Treatise on shoring & underpinning & generally dealing with ruinous & dangerous structures

THE LIBRARY

OF

THE UNIVERSITY
OF CALIFORNIA

LOS ANGELES

GIFT OF

John S.Prell

A TREATISE

SHORING AND UNDERPINNING.

A TREATISE

SHORING AND UNDERPINNING

AND GENERALLY DEALING WITH
RUINOUS AND DANGEROUS STRUCTURES.

JOHN S. PRELL

Civil & Mechanical Engineer.

SAN FRAtftJISCO, GAL.
CECIL HADEN STOCK,

THIRD EDITION, REVISED BY
FREDERIC RICHARD FARROW,

FELLOW AND GODWIN BURSAR OF THE ROYAL INSTITUTE
OF BRITISH ARCHITECTS.

WITH NUMEROUS ILLUSTRATIONS,

LONDON:

B. T. BATSFORD, 94 HIGH HOLBORN
1902.

Library

TH

PREFACE 10 THE THIRD EDITION.

THE very able manner in which this work was originally
prepared by the author has established it as the leading
authority upon the subject, and in revising it for a third edition
the present writer has, after careful consideration, found it
undesirable to re-write any considerable portion ; indeed it
would be impossible to do so without sacrificing something of
that clearness and conciseness which so strongly characterize
the book, and which render it of such great value in inculcating
the principles of Shoring and Underpinning, so far as is possible
without practical experience on the part of the student.

Extracts from the " London Building Act " of 1894 have
been introduced in place of those from the Act of 1855, in
force at the date of the first and second editions, which it has

Students and practitioners alike cannot be too strongly advised
to place the greatest reliance on the principle which the author
was one of the first to lay down, i.e., the infliction of the minimum
of disturbance to decrepit or dangerous structures in the practical
application of shores for their support.

The author's methods of reducing as much as possible the
number of needles and wedges employed are not universally
adopted, but even a slight consideration of the advantages
obtained by following his method should convince every student
of the superiority of the arrangements shown and explained in
this work.

FKEDERIC K. FARROW, F.R.I.B.A.

7, NEW COUBT,

LINCOLN'S INN.
June, 1902.

733400

JOHN S. PRELL

Ctoil & Mechanical Engineer.

SAN FRANCISCO, CAL.
PKEFACE TO FIEST EDITION.

THE object which the author of the following treatise has in
view is, as far as he can, to supply a want which has for some
time been felt among the younger members of the architectural
profession. It has been impossible hitherto, from the author's
own experience, to get up the subject of shoring and under-
pinning, whether as a necessary part of the education of an
architect, or for an examination, without a wearisome search
in different libraries for the scraps of information on the sub-
ject, scattered about among the works of various authorities ;
and the difficulty of obtaining information in this way has also
been considerably enhanced by the fact that two of the best
authorities on the subject write in a foreign language. Con-
sequently the student has been obliged, at a great sacrifice of
time, to fall back upon the expedient of sketching and measuring
existing cases ; an admirable method in its way, but which
would be more interesting and instructive (especially as what
one most wishes to know is very often hidden out of sight)
if some previous knowledge of the subject had been acquired.
Accordingly the following pages comprise a careful collection '
of all the authorities, together with a few additional notes and
sketches made from actual experience with the work.

The shoring and underpinning of the towers, columns, and
arches of mediaeval churches or other old buildings, which have
succumbed after having served their purpose well for many
years, is a subject too wide and complicated to be thoroughly
investigated in a text-book such as this ; a few examples how-
ever are given in Chapter VI., and methods are described in
which some one or two suppositional cases should be treated.

viii PREFACE TO FIRST EDITION.

Instances of this class are, however, comparatively rare, and so
varied in their character, that each requires to be treated in
its own peculiar way ; and it is impossible to lay down any
fixed rules, or to prescribe any definite methods by which
shoring and underpinning may be successfully carried out in
every case. But in the more general cases of shoring, such,
for instance, as are met with every day in London or other
large towns, where one house is so much like another in its
purpose and construction, it is possible, more or less, to prescribe
methods which will answer as well in one case as in another ;
and it is more the purpose of this book to explain these methods
and the rules involved in them, as they are more useful to the
student, and come into the everyday practice of the majority
of architects.

Shoring and underpinning, and dealing with ruinous and
dangerous structures, is one of the subjects of which a know-
ledge is required in the new examination for admission into the
Royal Institute of British Architects ; and the author has con-
sequently been careful to compile this treatise with a view, as
far as possible, to enable a student to answer any question that
may be set on this subject.

CECIL HADEN STOCK.
PARLIAMENT MANSIONS,
VICTOBIA STREET, WESTMINSTER,
May, 1882.

A TKEATISE

ON

SHOEING AND UNDEEPINNING.

CHAPTER I.
INTEODUCTOEY.

THEBE is, perhaps, no place where the principles of Shoring
and Underpinning ought more fully to be understood than
poor London, founded upon treacherous clay, built of bricks,
and abounding in ruinous buildings, where everything is done
in such a desperate hurry that anything that comes to hand
seems to be used as a building material by so many of our
builders, with, no doubt, the reflection on their part, " At all
events it will last our time." The delinquencies of the builder
and the treachery of the soil are, however, evils which are
common to most places ; and the student in the art of shoring
can hardly complain of a scarcity of examples to examine. In
London, at all events, he has only to turn sharply round the
corner of a street and he will run against a huge obstruction in
the middle of the footpath, the feet and sole-piece of a system
of raking shores. He will, doubtless, at once take out his note-
book and rule, and jot down the scantlings and the position of
the separate struts ; but when he comes to examine how the
whole system is wedged up, he finds that he is baffled by the
sole-piece being so buried in clay and dirt that he can elicit no
definite information from it. This is nearly always the case

S. B

2 SHORING AND UNDERPINNING.

when work is examined without any previous acquaintance
with its principles ; and it is the desire of the author to instil
into the mind of the reader a sufficient acquaintance with the
principles and the terms used in shoring and underpinning,
that he may afterwards with confidence perfect himself in the
practical knowledge of the subject by sketching and measuring,
and by questioning foremen engaged upon the work.

It is always necessary, in attempting to obtain information
from a workman, to go well armed with terms ; for as a rule he
takes it for granted that you understand the phrases he uses,
and vouchsafes no explanation concerning them. However,
this is always the best way to gain practical knowledge upon
anything : see the work begun and carried out to the end, go
into its object, criticise it if possible, and consider whether,
from your knowledge of the subject, it could not have been done
better some other way. No student should be content with the
knowledge he had gained simply by reading a book.

The mathematical investigation of the nature of the forces
brought into play in the case of raking shores, though it can
hardly be said to be absolutely necessary, is still well worth
the attention of the reader ; for it gives him an altogether
superior grasp of the subject, and makes him feel competent to
undertake the most difficult problem it can afford. The in-
vestigation of the nature of strains comes, of course, into many
other of the studies of an architect, and the time spent in con-
sidering the proof of formulae employed can never be said to be
spent in vain. But, for the convenience of those who may not
be acquainted with the science of trigonometry or statics, the
theoretical has not been allowed to interfere too much with the
practical side of the subject ; and the mechanics of raking shores
have been banished to a chapter by themselves at the end of the
book, so that those who do not understand them need not trouble
themselves to read them at all.

As there are now so many and varied subjects connected with
our profession, making it almost an impossibility to gain a
sufficient practical knowledge of all of them, there is every

INTRODUCTORY. 3

reason to believe that there will be in the future a demand for
specialists : that is to say, an architect having on hand some
work which has not before come much within the range of his
practice, might be glad to consult with some other member of
his profession who had made a special study of that one par-
ticular subject. And so, if any of the readers of this book feel
that they have any inclination to make a special study of shoring
and underpinning, and generally dealing with ruinous and
dangerous structures, they will find it a pleasing and interest-
ing pursuit, having an element of danger and excitement about
it sometimes which gives it a superior charm over many other
of the architect's duties.

When once the study and practice of shoring has been
acquired, there will always be found, especially in London,
ample scope for the specialist in this branch of the profession
to exercise his ingenuity ; for there is scarcely ever a house
cleared away for the erection of a new building without its being
necessary to shore up its neighbours on either side. And so this
subject cannot be too well understood by all architects, for many
accidents occur from the shoring being left to the rule-of-thumb
of the foreman employed on the works, without any supervision
by some more responsible person.

We shall now proceed to describe the different methods used
in shoring and underpinning, taking first into our consideration
the ordinary every-day cases to be met with in London and
other brick-built towns.

(4)

CHAPTER II.
ON BAKING SHOEES.

WE shall describe in this chapter only the ordinary use of
raking shores, reserving the different varieties of this method
to be considered by themselves in another chapter.

In Plate I. Fig. 1, there is depicted an example of the
raking shore in its most simple form, i. e. with only one
principal strut. Let us suppose it to be supporting a brick-
wall, 9 inches thick and 20 feet high from the ground, A C ;
then A B is the principal strut, called a shore. E is a deal,
called the wall-piece, 9 inches wide and 3 inches thick, and
long enough to take the foot of the secondary strut G. In
this wall-piece, about 2 feet from one end, a rectangular hole
is cut and a small piece of wood D, called a needle, or by
some workmen a tossle, or joggle, is inserted, projecting about
4£ inches on either side of the deal. A half header is taken
out of the wall near the top and the wall-piece placed in
position, the needle fitting into the hole thus prepared. The
other end of the needle, projecting beyond the face of the
wall-piece, serves as an abutment to the head of the shore
at B. For additional security a wedge-shaped piece of wood,
C, called a cleat, is nailed on to the wall-piece just above the
needle, and prevents it from being forced out of its place by
the upward pressure of the shore A B. F is a balk of timber,
called the footing block, or sole-piece, let into the ground, or
if the ground be soft, laid upon a small platform of timber.
A cleat is nailed upon the upper side of the sole-piece at A
to prevent the foot of the shore from slipping. Ah1 these
parts in connection with the shore will be taken more in detail
further on in this chapter: at present let it suffice only to
name them and describe their functions.

ON RAKING SHORES. 5

Now, the object of this shore is to prevent the wall from
being turned over by the thrust caused by a house leaning
against it. In considering the resistance to be offered to this
thrust, though it may but seldom be the case, yet we must
always be prepared for it at its greatest magnitude, and that
will be when it is great enough to upset the wah1. The
direction of this thrust will of course be at right angles to
the wah1, and it will act at a point near the top, i. e. where
the head of the shore presses against the wall, as shown by
the line Q in the figure. Now, the most convenient way to
overcome such a thrust, would be, of course, to place a strut
at right angles to the wall, or in other words, in a line with
the direction of this thrust Q (as in the case of a flying
shore). But when this cannot be done a raking shore is
necessary, and it is easy to see that as soon as the position of
the strut is altered from a horizontal to an inclined position,
a certain percentage of force is wasted in an endeavour to
thrust up the wall ; so, taking into consideration the fact that
action and reaction are the same, there may be said to be two
forces brought into play in the case of a raking shore, one the
thrust (Q) of the wall, and the other a force P, exercised ver-
tically by the weight of the wall above D upon the head of
the shore, tending to keep it down, in which the weight of
the shore itself must be taken into account. The two forces,
acting at right angles to each other at the point B, must have
by the law of forces an equivalent single force acting in some
direction between the angle of the directions of these two
forces ; and this force is called the resultant. Now, this resul-
tant does not act down the direction of the shore itself, but in
some direction which varies as there is more or less thrust
(Q) of the wall ; and this direction is found by mathematics to
be always outside the angle which the shore makes with the
horizon, as the line p A ; consequently the balk or sole-piece
must not be put at right angles to the direction of the shore,
but at an angle, as near as can be judged, at right angles
to a mean of all the directions the resultant may take ;

6 . SHORING AND UNDERPINNING.

and this will be, as near as possible, at right angles to the
line p A.

Now in practice this truth is of the greatest convenience,
for the foot of the shore, being gently levered along the sole-
piece, is compressed tighter and tighter, and so the necessity of
wedges is dispensed with entirely. In order to facilitate this
operation of levering the shore into its place, a groove or slot is
cut in the under side of the foot of the shore, large enough only
for the carpenter to insert the end of a crowbar as a lever. (See
Plate II. Fig. 6.) This was the method employed in most of the
shoring executed for the Metropolitan Board of Works, and it
may be considered the best ; for, in dealing with structures that
are really in danger of falling, the greatest care must be taken
to avoid all blows with a hammer or mallet, such as are
necessitated by the use of wedges at the foot of the shore.

From what has been said about the tendency of the shore
to lift the wall, and the consequent reactionary force P, which
keeps the head of the shore down, it is obvious that the needle
must not be placed too near the top of the wall, for, unless
there is sufficient weight upon its head, the shore will rise and
burst up the courses above it.

There is yet another force brought to bear upon a raking
shore which we must not forget to mention, and that is a
cross strain, S, acting at right angles to the shore, and tending
to bend it inwards, the truth of which may be investigated in
the chapter on the Mechanics of Eaking Shores, at the end of
the book. It is to counteract this cross strain that the
secondary strut G is necessary, and in cases where three or
four shores are combined in one system, as in Figs. 2 and 3,
this strut answers the double purpose of counteracting the
cross strain and binding the shores together.

Having now considered briefly the nature of the forces brought
into play in the case of this single raking shore, and the practical
lessons that they teach, it only remains to be said that where
there are any number of shores in a system, each separate shore
in that system is subjected to the same forces or strains as have

FIG I .

J Akennan,Hioto-litk Londa

: Inert M .

ON RAKING SHORES. 7

been described in the case of Fig. 1. In fact, this figure may
be considered as the outer shore in a system of two or more
shores. It has been used here more for convenience of descrip-
tion than as a method to be adopted. As a general rule, two or
more shores should be used in a system.

With regard to the scantlings of the timbers used in raking
shores, as they are for the most part but temporary erections,
builders generally use such timbers as they may have by them,
which are too rough for better work. But in order to be quite
sure that the timbers are strong enough to resist the utmost
strain that can be put upon them, it is always as well to use the
formulae, which are appended, with an example, at the end of
this chapter.

We will now go on to consider Fig. 2 in Plate I. This is the
raking shore most commonly seen ; it is simply a triple arrange-
ment of that described in Fig. 1. The wall-piece is made much
longer in consequence, and has three holes cut in it, and three
needles inserted with their cleats nailed above each. The outer
shore is called the top raker, the middle shore the middle raker,
and the lowest is called the bottom shore. As the top raker of
this system is a much longer shore than that shown in Fig. 1,
it will be necessary to strengthen it with more than one
secondary strut. This is done by nailing pieces of timber about
1 inch thick, and from 6 inches to 9 inches wide on either side
of the shores, as shown at G in Figs. 2 and 3. These braces
are brought home against the wall, and nailed to the sides of the
wall-piece (which, if wider than the shores, is best notched out
to receive them), and their position is generally just below the
points where the needles enter the wall. As the bottom shore
cannot conveniently have a secondary strut, it is generally tied
up by a brace similar to those at G ; this brace is also useful to
bind the three shores together as they approach each other at
their feet, and helps to render the whole a homogeneous system,
incapable of turning about or warping when tried by the thrust
of the wall. Hoop iron is also nailed round and round the feet
of the shores to prevent any possibility of their separating.

8 SHORING AND UNDERPINNING.

In Plate I. Fig. 3, we have a much larger and more com-
plicated system of shores. It differs from the other in this
respect, that the top raker or rider shore, as it is called in this
case, instead of coming down to the ground as before, is made
to spring from the back of the shore immediately below it.
This is, of course, done because it would be impossible, except
at considerable expense, to obtain so long a piece of timber as
would otherwise be required. In some cases the foot of this
rider shore is made to rest upon a large cleat, nailed to the back
of the shore below, but the best method is to let it rest upon
another piece of timber of the same scantling, which, secured to
the back of the shore below, goes down to the sole-piece, as
shown in the Fig. 3. This rider shore may be of a smaller
scantling than the others.

Now this plan, though it answers very well in a case like
Fig. 3, should not be allowed in the case of Fig. 2 (unless there
was great difficulty in obtaining a piece long enough for the
top raker), for this reason, that the power of wood to resist
compression is very much impaired by any cross strain that may
be put upon it. But still, if in the case of Fig. 2 the house was
really in imminent danger of falling, or was very much out of
the perpendicular, it would not only be advisable, but even
absolutely necessary, to keep the top raker in two pieces, and
fix it as in Fig. 3, because the disturbance and blows upon the
wall, which would be caused by the moving about and fixing
so large and heavy a piece of timber, might result in bringing
about what it is the object of the shores to prevent, viz.,
the total wreck of the house. But whenever the method of a
rider shore is adopted, the shore below it must be made
proportionately stronger, to enable it to resist the cross strain.

This may be done either by increasing the scantling of the
raker, from the back of which the rider shore springs, or by
solid struts between the rakers, as shown in Fig. 1, Plate II.

In Fig. 2, Plate II., is shown an example of an extended
application of this latter method for a lofty building of several
floors.

PLATE, III.

Fig: 6.

C.Hcultn Stock Aiv. ttlttl.

ON RAKING SHORES. 9

When very long timbers are required for the rakers, they are
sometimes scarfed. The scarfs should be square shouldered,
with fish-plates of iron or oak plank, and should be made with
great care.

In those cases where it is of considerable importance that
the shores should be kept as light as possible, a great reduction
of weight may be effected by trussing the timbers with cast-iron
or hard wood struts, and wrought iron or mild steel tie-rods.
The trussed timbers are, however, more expensive, and though
lighter in weight are more awkward to handle, so that they are
rarely employed in shoring.

Whenever possible, the method described for Fig. 1 of levering
the shores into their positions on the sole-piece should be used ;
but if it is found impracticable to compress sufficiently the
middle rakers in this way, oak wedges can be used, care being
taken to drive them home gently, the object being to support
the wall, not to thrust it over. The rider shore is compressed
by two oak wedges, gently driven home on either side of the
foot where it meets the timber secured to the shore below, as
shown in Plate III. Fig. 1. The system of shores which has
only two struts is a very common one, its principles and con-
struction being in every way similar to what we have already
described.

The following paragraph may be taken, as a general rule,
for the number of shores to be employed in each system, and
the scantlings that should be given to each : —

For walls from 15 ft. to 30 ft. high, two shores are necessary in each system
Ditto 30 ft. to 40 ft. „ three ditto.

Ditto 40 ft. and upwards, four ditto.

Taking the angle of the shore at 60° to 75°,
For walls from 15 ft. to 20 ft. high, 5 in. X 5 in. may be the scantling for

each shore.

Ditto 20 ft. to 30 ft. „ 6 in. X 6 in. ditto.

Ditto 30 ft. to 35 ft. „ 7 in. X 7 in. ditto.

Ditto 35 ft. to 40 ft. „ 8 in. x 8 in. ditto.

Ditto 40 ft. to 50 ft. „ 9 in. X 9 in. ditto.

Ditto 50 ft. and upwards, 12 in. X 9 in. ditto.

10 SHORING AND UNDERPINNING.

The systems of shores should not if possible be more than
from 12 feet to 15 feet apart ; but if they are placed nearer to
each other than this the scantlings may be made lighter, which
will be of great advantage in the case of a really dangerous
structure for the reasons mentioned above.

The general arrangement and construction of the raking
shores which are most commonly used having been explained,
it now remains only to say a few words with regard to their
details. As these can best be explained by the figures on
Plate III., it will be sufficient merely to refer to them, pointing
out their uses.

The meaning of Fig. 1 has already been explained ; the oak
wedges, which compress the rider shore, have been driven
home, and sawn to a neat appearance. Fig. 2 is the needle,
which in good shoring is made out of a piece of wood about
4 inches square and 1 foot long, cut down at one end to the
size required to fit the hole made in the wail ; a shoulder is
thus formed to butt against the wall-piece, and a good strong
abutment is afforded for the head of the shore. Fig. 5 is
a sketch showing the needle in position in the wall-piece, with
the cleat above it, and the manner in which the head of the
shore is notched to fit the under side of the needle. This is a
very necessary expedient ; for the author has known an instance
of the top raker in a system of shores, a long and heavy piece
of timber, having been blown down by a sudden gust of wind
— seriously injuring two workmen who were underneath it at
the time — from the neglect to notch the head of the shore, or
otherwise secure it in case of its becoming loose. Iron braces,
as Fig. 3, called iron dogs, are sometimes used for this purpose,
as also for securing the feet of the shores to the sole-piece, and
the foot of the rider shore to the shore below it. It is important
that the pointed teeth of the iron dogs should be at right angles
and not at an acute angle with the shank. It will be noticed
that, in the sketch Fig. 5, the wall-piece is secured to the wall
with iron hooks, a detail of which is shown in Fig. 4 ; these
are convenient to hold it in position during the insertion of the

ON BAKING SHORES. 11

needles and fixing of the shores. Fig. 6 shows the method of
levering the feet of the shores along the sole-piece. A cleat
should be nailed to the sole-piece against the foot of the outer
shore, and the spaces between the shores, if they do not quite
touch, should be filled in with a bit of stuff.

The greatest care • must be taken that the sole-piece rests
upon a firm foundation of solid ground, for the efficacy of the
shore depends very much upon an unyielding base. It should
first be ascertained that there are no cellars or vaults under the
spot that the sole-piece is to occupy, and all made ground, soft
clay, &c., should be avoided if possible ; but if, as often happens,
it is impossible to obtain a firm foundation without going to a
considerable depth, the sole-piece must rest upon a carefully
made platform of timbers, laid across each other, which will
press equally upon the ground all over. This platform may be
laid level, and the sole-piece raised to the required inclination
by wedge-shaped pieces of oak fixed upon it ; or it may be laid
at once to the inclination of the sole-piece. In some cases,
where great pressure is likely to come upon the shore, a good
bed of concrete is prepared, and the platform which takes the
sole-piece laid upon it.

When a space has been cleared away, and raking shores
are erected to support the surrounding buildings, they must not
be put up indiscriminately, without reference to the plans and
sections of the new building which is to occupy the space ; but
care must be taken that they shall interfere with the building
operations as little as possible. As the new building rises to
the under side of the bottom shores, they are taken down, the
middle and top rakers being left in position till they are reached
in their turn. The foundation of the shores should be left
untouched until all are taken down.

It is obvious that the more inclined the shore is with the
horizon, the greater is the lateral thrust it will exert. An
angle of 40° is considered to be the best inclination for raking
shores ; but there is very seldom room for so great a spread at
the foot as this requires, and they are more often raised to an

12 SHORING AND UNDERPINNING.

angle of 60° or 70°. But it should always be borne in mind
that the more the shores are brought in at the feet, the less will
be the lateral thrust they will exert against the wall.

Formula for determining the pressure brought to bear upon a

raking shore by the house which it supports.
Eef erring back to the example (Plate I. Fig. 1), we have seen
that there are two principal forces, P and Q, brought to bear
upon the shore. Before we can determine what effect these
two forces have in compressing the shore, we must first
discover the magnitude of each of them separately.

To find the maximum horizontal thrust Q exercised upon the
wall (Q being in cwts.), we must use the following formula : —

Where W is the weight of the wall in cwts., t is the thickness of
the wall at the ground line in feet or parts of a foot, and B C is the
distance of the head of the shore from the ground, also in feet.
To find the vertical force P, to be expressed in cwts. : —

P=Qtan. 0*-| ..... II.

Where 0 is the angle the shore makes with the horizon, and w
is the weight of the shore itself in cwts.

Having found by the above formulae the values of Q and P,
we can now find the compression F down the shore which they
produce, by the formula —

F = P sin. 0 + Q cos. 0, ____ III.

where F is in cwts. and 6 is the angle the shore makes with the
horizon.

* The reader, even if he has never before been acquainted with the trigo-
nometrical symbols, sin., cos., tan., &c., need not be at all alarmed at their
appearance here ; for if we know how many degrees there are in the angle 0,
we have only to refer to some table of natural sines, cosines, tangents, &c., and
we shall find that the expression tan. 0 is transformed into a convenient
decimal. For instance, the shore A B in PL I. Fig. 1 makes an angle of

ON RAKING SHORES. 13

Now, taking the example in Plate I. Fig. 1, a wall 20 feet
high, with a frontage of 10 feet, is supported by a raking shore
of fir, 4 inches by 4 inches, the head of which B is 16 feet from
the ground AC. The angle 6, or B A C,'is 70°, and by referring
to a table of natural sines, &c., we find that sin. 0 is -93969,
cos. 0 is -34202, and tan 0 is 2-7474. Taking the waU at
1 cwt. per cubic foot, its weight W will be 150 cwt., and its
thickness t is 9 inches, or f of a foot. The weight w of the
shore itself is -f cwt.

The value of Q is obtained from I.

Wx* 150 xf 112$. ,
Q = 2lfC -- 32! = ~32 = * approximately,

i. e. the maximum horizontal thrust is 3| cwt.
The vertical pressure P is obtained from II.

P = Q tan. 0-|= (3* x 2-7474) -^-9* cwt.

The compression F down the shore is found from III.
F = P sin. $ + Q cos. 6 =(9J x -93969) + (3$ x -34202) = 9f cwt.
approximately ;

i. e. the shore as a post has to resist a pressure of 9£ cwt.

To find whether the shore is strong enough to resist this
pressure, we must use the formula for a long, square post,

L-.x*

where a is 15-5 for fir, d is the least width in inches, and I is
the length in feet ; L being the safe load in cwts. that may be
put upon it.

Very often, however, the breadth of a shore is double its
width, i. e. the sides of the section bear a proportion to one
another of f- ; consequently, as we get twice the resistance, we
multiply L as found by the above formula (in which d in this
case is the lesser side) by 2 for the safe load required.

70° with the horizon, A C. We look, say in Molesworth's " Pocket-book of
Engineering Formulae," for the tables of natural sines, &c., and we see that
tan. 70" is 2-7474.

14 SHORING AND UNDERPINNING.

When the section is 6 inches by 4 inches, for instance, the
sides bear a proportion to one another of -| or |, consequently
we multiply L as found by the above formula (in which d is
the lesser side) by -| for the safe load required ; and so for all
scantlings.

It has, however, been found by experience to be always best
to make shores of square timber. The shores erected by the
London County Council are generally of a square section, and
timber of this kind can easily be obtained from 4 inches by
4 inches to 13 inches by 13 inches.

In the example before us the shore has a square section ;
then,

L = 15-5 x Iff = ISfcwt.;

i. e. the safe load which the shore will carry is slightly in excess
of the compression E, to which it is actually subjected.

In this manner the top raker in a system of shores can be
tested.

The compression down the middle raker and bottom shores
can also be determined in the same way, a separate value of Q
being worked out for each. It will be found that this com-
pression increases as the shores are placed lower down the wall ;
but as the power of resistance in the lower shores is also con-
siderably increased from their being so much shorter than the
top raker, they will be quite strong enough if made of the same
scantling.

PLATE, IV.

Fig 3

JAk«rmt» Photo -lith London ,WC.

CHAPTER III.

ON A FEW ADDITIONAL PEINCIPLES, VAEIETIES,
AND USES OP THE BAKING SHORE.

THOUGH we but seldom see any other modifications of the
raking shore than those which have already been described,
yet there are certain cases which call for special adaptations of
this system ; and it is for the carpenter to prescribe that method
in each case, which his skill suggests or his experience dictates.
It would, of course, be needless to describe every variety and
use of the raking shore ; but some few additional remarks (for
which, with the diagrams, the author is indebted to M. Viollet-
le-Duc's article on Shoring), may possibly be useful.

It is usual in most cases, where more than one shore is
used in a system, to spread them out at the top and bring them
together at the foot ; but this should not be done if there is a
sharp bend in the wall, as at C in Plate IV. Fig. 1, but the shores
should be placed as is indicated in the figure, i. e. they should
be farther apart at their feet than at their head. For (as it is
necessary, in all cases where there is a prominent bend or
rupture in a wall, that the head of the outer shore should rest
exactly above the point where this bend or rupture occurs),
if the usual method be adopted the head of the lower shore will
act at the point E ; but it will be dangerous to exert a hori-
zontal pressure upon the wall at this point, for it will only tend
to aggravate the rupture below the bend at C. But when a
wall is bent in a uniform manner as in Fig. 2, the shores are
best placed in the usual way, approaching together at the foot ;
for while the upper shore A B takes the load, the lower shore
E B can exercise a more effectual resistance to the bend of the
wall. Thus it is always best to employ more than one shore in
a system.

16 SHORING AND UNDERPINNING.

Fig 3 shows the way in which the head of a raking shore
may be fixed in a masonry wall. A hard header of stone is
built into a hole made in the wall, projecting beyond the face
of the old work, and a piece of heart of oak is placed underneath
it as a seating or needle for the head of the shore.

When two or more shores are employed in a system they
should never be parallel, but (to quote M. Viollet-le-Duc), " they
should always form a triangle or a portion of a triangle, for
this reason, that a triangle can never be thrown out of shape ;
when braced, shores which are not parallel present an entirely
homogeneous resistance ; whereas if they are parallel they will
become bent, however well braced they may be."

This truth may be extended further ; and when two systems
are combined in one, as is sometimes done when great strength
is required, they should not be placed parallel to each other,
but they should form a triangle on plan, as is shown in Fig. 4
in perspective. This kind of shore, if it is well braced, is exceed-
ingly strong, and suitable to prevent the pressure of the earth
from overturning a terrace wall.

It is often possible to make use of a raking shore, not only as
a support, but also as a means whereby a wall may be pushed
back again from a leaning to an upright position. An instance
of this has come under the author's notice in the case of one of
the walls of a large warehouse, which had gradually been pushed
out of the perpendicular. The foundations were examined and
found to be in a comparatively good condition, and the face of
the wall, though out of the perpendicular, presented a uniform
appearance, i. e. there were no serious bulges or cracks percep-
tible on its face ; and consequently the idea of restoring its
perpendicularity seemed possible to be put into execution,
without any danger to the wall itself. Accordingly, raking
shores were placed at intervals along the wall, and a powerful
screw jack fitted under the sole-piece of each system ; the con-
nections of the quoins with the return walls were then cut
away, and the roof and floors of the buildings, having first been
propped up with posts and struts from the basement to the

ADDITIONAL USES OF THE BAKING SHORE. 1?

topmost story, were also cut off from all connection with the
wall. A wedge-shaped fissure was next cut in the brickwork,
at a point near the base of the wall on the internal face, and
the space thus cut out was filled up with sand. The screw
jacks were turned evenly and gently, and the wall, squeezing
the sand out of the fissure, was gradually pushed back by the
shores into its original position. The roof and floors were
again firmly connected with the brickwork, the posts and
shores taken down, and the whole then presented an
appearance as strong and satisfactory as when it was first
erected.

Another method used for bringing back into the perpendicular
the two opposite walls of a building, which have been thrust
outwards by a roof or vault, though it is perhaps hardly d
propos here, may as well be mentioned. It consists in fixing
bars of wrought iron across the building from one wall to the
other, which pass through to the outside, and are then screwed
to large nuts, or washers, placed against the external face of
the walls. Fires are lighted under these bars, and as the metal
expands the washers are screwed up as tightly as possible. The
fires are then extinguished, and when the bars begin to cool,
the force of their contraction gradually draws the walls
together.

But to return to the raking shore, another of its many uses
is to steady a wall whilst it is in process of being underpinned ;
these raking shores should be left in position for some time
after the works have been carried out, so as to enable the
wall to take its bearing upon the new work without danger of
disruption.

The best wood in which all shores should be made is un-
doubtedly the fir, because its grain is always straight, and it
can be obtained in long pieces. It is difficult to make good
shores of oak, as it is generally of a middling length, has a
twisted grain, and is heavy and more troublesome to lift in
consequence. Oak ought to be used, however, in preference to
all other woods for the wedges, seatings, &c., and even for the

S. C

18 SHORING AND UNDERPINNING.

sole-piece (though this is seldom done), because its texture does
not crush under the load like that of fir.

Care should be taken that the shore is thoroughly well put
together, that all the joints are made to fit exactly, and that the
foot of each strut has a perfect bearing upon the sole-piece.
Nothing is more satisfactory than to see a shore well made, and
those who design and construct in this art cannot help feeling,
in such a case, a pang of regret when their handiwork is cleared
away.

PLATE, V.

C.ffasi*,, .Sta/AJn*-.

J.Akerni»ti i-hoto-fcth.London.V.'.C.

(19 )

CHAPTER IV.
ON HOEIZONTAL OE FLYING SHOEES.

WHEN a house is taken down in a street, and the party walls of
its neighbours on either side require supporting, and if the space
between the two is not greater than about 32 feet or 33 feet,
horizontal struts, reaching from one wall to the other, are
employed ; these are called flying shores.

In Plate V. Fig. 1, is depicted the usual method of con-
structing these shores. Two wall-pieces B are provided and a
rectangular hole cut in the centre of each for the insertion of
the needles D, which rest in holes cut in the walls, just as has
been described in the case of raking shores, a cleat C being
nailed below them for additional security. The horizontal
strut A B is compressed by oak wedges driven together above
the needles D, and it is stiffened by the raking struts G, which
butt against cleats C on the wall-pieces, and against straining
pieces F, securely nailed to the top and under side of the
horizontal strut A B.

It will be easy to see that by this method a very effectual
resistance is offered to any inclination of the houses to fall in
upon each other ; but it will also be necessary, in most cases
where flying shores are employed, to support the angles of the
walls towards the street with raking shores, as shown at H in
Fig. 1. Of the two houses, however, here represented, the one
on the left hand is secure, and needs no shoring at all, having
been built independently of the house that has been cleared
away, or in other words, the return wall belongs to it exclusively,
and has not been shared as a party wall by the house adjoining ;
consequently the flying shore has to resist the thrusts of the
opposite house only. But when both are party walls it will be
best, although not theoretically necessary, to allow sufficient

c 2

20 SHORING AND UNDERPINNING.

strength in the shore to resist the thrust of both the houses
together ; and it will also be necessary to support the angles of
both the walls, both in the front and at the back, with raking
shores.

The thrust exercised by the wall of the house on the right-
hand side in Fig. 1 may be found at any point in its height by
the useful formula,

Wx*

Q=2BC'

where Q is the thrust in cwts., W is the weight of the wall in
cwts., t is the thickness of the wall at the ground in feet or
parts of a foot, and B C is the distance, in feet, from the ground
to the point at which it is desired to ascertain the thrust.

It is obvious from the laws of leverage, that the best position
for the shore to occupy is near the top of the wall, as shown in
the figure ; and by working out examples by the above formula,
which is framed on the supposition that the wall is just falling,
it wiU be found that the thrust will increase considerably as
we come lower down the wall. Consequently, if from some
inconvenience, the shore cannot be placed near the top of the
wall, it must be made proportionately stronger the lower it is
brought down. It is a common and a good practice to place
two or more flying shores one above the other, in the same
perpendicular plane, thus holding up the wall at every point in
its height. In this case it is best if possible to have the wall-
pieces in one length from the top to the bottom of the system.
If the wall bulges at certain points, as in the figure, or if any
projections occur upon its face, the wall-piece must be packed
up behind with firring pieces, and so made to press equally
against the wall at every point in its length.

The reason why the span of a flying shore was limited,
apparently so dogmatically, at the commencement of this
chapter, to 32 feet or 33 feet, is because ordinary Dantzic fir
cannot easily be obtained in pieces of a greater length than
this. Scarfing or joining two lengths into one is not a wise
practice in the use of flying shores ; for unless the scarf is

ON HORIZONTAL OR FLYING SHORES. 21

executed with greater care than is warranted by the temporary
character of the work, it is worse than useless. For it cannot
be guaranteed that the horizontal strut will only be compressed
in the direction of its axis, but the wall, if it leans forward
uniformly, will make its thrust first felt down the upper raking
struts, and so produce a cross strain upon the principal strut ;
and the power of the principal strut to resist this cross strain,
even though it is stiffened by the lower raking struts, would be
very much diminished by a scarf, especially if the span is con-
siderable, as of course it would be if a scarf was necessary.
Thus it is always best to use only one whole piece of timber
for the principal strut ; and if this cannot be obtained long
enough in Dantzic fir, pitch pine must be used, which can be
procured in pieces, if necessary, 66 feet long. But unless the
houses to be shored are a great height, say from 70 feet to
80 feet high, it would be more economical to make use of
raking shores.

With regard to the scantling that should be given to the
timbers of a flying shore, the following may be taken as a
general rule : —

For spans not exceeding 15 feet, the scantling for the prin-
cipal strut may be 6 inches by 4 inches, and for the raking
struts, 4 inches by 4 inches.

For spans from 15 feet to 33 feet, the scantling for the
principal strut may be from 6 inches by 6 inches to 9 inches by
9 inches, and for the raking struts from 6 inches by 4 inches to
9 inches by 4-J- inches.

In both cases the straining pieces must be stout enough to
give a good bearing to the ends of the raking struts.

The scantlings given above are for shores which occupy a
position at about three-fourths of the distance from the ground
line to the top of the wall, and which are placed at intervals of
not more than 10 feet to 15 feet from each other.

It may sometimes happen that when it would be more con-
venient and economical to support a house with flying shores,
an objection is raised by the owner of the house which it is

22 SHORING AND UNDERPINNING.

proposed to use as an abutment, either because he is afraid of
his wall being pushed in by the pressure brought to bear upon
it, or because the unsightly appearance of the shores may be
prejudicial to his premises. This objection he has a perfect
right to make, and he can compel his neighbours — of course at
his own risk — to tie in the wall from the back, or, if there is
room on then" property for the erection of raking shores, to
adopt this method of supporting the wall. There was an
instance, some years ago, of a case of this kind on the Thames
Embankment, opposite the Temple Station of the District Kail-
way, where, although the wall of the house which required
support was over 60 feet high, and there was an admirable
abutment for flying shores close at hand, yet the more expen-
sive method of raking shores was adopted, no doubt because
the adjoining owner objected to have his premises disfigured,
as they certainly would be by flying shores butting against
them.

We will now go back to consider Plate V. Fig. 2. This is a
contrivance which must be employed if the house to be sup-
ported is higher than the house which is used as an abutment.
It is more convenient, more economical, and more effectual
than a raking shore springing from the ground would be,
especially if the height of the building is considerable. In fact,
in all cases where the span is not more than about 33 feet, and
there is no difficulty in obtaining a good abutment, it is always
best to employ flying shores in preference to raking shores ;
for, apart from the consideration of economy, they present a
more direct resistance to the thrust, are well out of the way of
any building operations that may be carried on below them,
and can remain in position without danger of being disturbed ;
whereas the feet of raking shores are always in the way, and
the excavating and pumping which is so often carried on around
them, unless great care is taken, is almost sure to loosen their
foundations, and so to render them useless.

CHAPTER V.
ON NEEDLE SHOEING AND UNDEKPINNING.

WE have hitherto been dealing only with those methods of
shoring which are used more particularly when some pre-
cautionary measures must be taken to arrest a dangerous
movement in the wall of a building, but which may be said only
to assist the foundation in the real task of supporting the wall.
We now come to consider the case when the support of the
foundation is no longer to be relied upon, and the wall is to be
gripped and held suspended in the air by the shores alone,
while its lower portion is cleared away entirely, either to be
replaced by new work or to remain open for a doorway or shop
front. The method employed to support the wall in such a case
is called needle shoring : in principle it is the most simple of
any, and needs but little explanation ; but in practice it requires
the greatest care.

It consists merely in cutting holes about 14 inches square
through the wall of a building, at intervals of from 5 feet to
7 feet from each other, and inserting through these holes short
balks of timber called needles, which are propped up at either
end by stout posts, resting upon sole-pieces laid upon timber
platforms on the ground. Oak wedges are driven together at
the feet of these posts, or the sole-pieces are laid at a slight
inclination, and the posts are levered into position in the same
way as the feet of raking shores. The needle is thus pressed
tightly against the under side of the brickwork, and after
raking shores have been fixed as an additional security in
supporting the wall, the lower portion can be taken down with-
out fear ; the whole weight of the wall and floors being carried
by the needles, and transmitted through the posts to the ground.
The wall is supported on the principle of a corbel springing

24 SHORING AND UNDERPINNING.

from either side of the needle, and finding its way through the
perpendicular joints, until it is met by the line of the corbel
springing from the neighbouring needle. It might be supposed
from this that the triangular space between the corbels, having
nothing to support it, would fall out ; but this is not the case in
practice, for the adhesion of the mortar is sufficient to hold all
the bricks together if the distance from one needle to the other
is not greater than about 6 feet or 7 feet. However, if there is
any tendency on the part of the bricks in this space to fall, they
must be temporarily strutted up from below. In this kind of
shoring there is nothing to be gained and everything to be lost
by using timber of a small scantling. For the needles, and for
the posts as well (unless they are very securely braced to each
other), whole timbers (i. e. about 13 inches by 13 inches) should
be used.

The above brief description brings us to an end of the three
methods of shoring usually employed to support a building ; but
before we give a practical example of this last method, it may
be as well to say a few words here upon the general subject of
ruinous and dangerous structures.*

The first thing of course to be done when a structure is found
to be unsafe is to shore it up at once on all sides, either with
raking or flying shores, as may be most convenient ; but, before
we can determine how it can best be restored to a sound con-
dition, a careful survey must be made of all the walls, so that
we may find out from the nature of the cracks and bends, and
other guiding marks, what is the cause of the failure, and in
what direction the fault lies ; for in this way only can we know
with certainty how and where to apply a remedy. There are,
of course, many causes to which the failure may be attributed,
all of which should be considered when the building is
examined, such, for instance, as the use of bad mortar, the over-
loading of the wall, the thrust of a vault, or, more commonly,

* For the convenience of the London reader, the law concerning
dangerous structures in the metropolis is appended at the end of this
Chapter.

ON NEEDLE SHORING AND UNDERPINNING. 25

some defect in the foundations.* But as it would be an
impossible and useless task for us to go into all tbe cases of
failure that are likely to occur, and to prescribe here what
should be done in the way of remedy in every instance, we
must content ourselves with the investigation of one example
only, and let it suggest in principle at least what should be
done in many other cases.

The failure of the foundations is, as we have said above, the
most common cause of ruin in a building, and the method of
restoration known as underpinning, which is employed in such
a case, is of every-day occurrence ; consequently we cannot do
better than select this subject in considering the treatment of
ruinous structures.

If, after a thorough examination has been made of a dangerous
building, and from the nature of the cracks and bends and
other evidence of failure in the walls, it is proved beyond doubt
that the fault is not to be found in the superstructure, then
inspection trenches should be cut, and the foundations examined.
At one time it may be discovered that the footings have been
built with bricks or stones which are both bad in themselves
and improperly bonded in the work ; for this is, unfortunately,
a very common practice with some builders, to get rid of all
their bad bricks, or odd bits of stuff, in the work below the
ground. Nothing leads to more disastrous results ; for it should
be remembered that the lower a stone or brick is placed in a
wall the greater is the weight it has to carry, and consequently
the very best materials should be used in the foundations of a
building. At another time it may be found that the footings
have buckled up at either side into the shape of the letter V,
from the offsets being too great, or from the fact that back joints
have been allowed beyond the face of the upper work. Only
heading courses should be allowed in footings, stretching courses
should only be used when the footing courses are doubled, and

* The reader is referred to an excellent paper, by Mr. Edwin Nash, on
" Failures in Construction," recorded in the " Transactions of the Eoyal
Institute of British Architects," 1867, vol. xviii.

26 SHORING AND UNDERPINNING.

then the stretching course should occupy a position under the
heading course. It may often happen that the concrete or the
mortar used for the brickwork below the ground, if the situation
is a very damp one, has never properly set from the want of
hydraulic properties in the lime used ; or the concrete and
foundation generally may have been dislocated by the expan-
sion and contraction of the clay on which it rests. Again, the
failure may be caused by some defect in the design of the wall
below the ground, such for instance as piers standing upon
inverted arches which have not sufficient abutment ; or if the
building be an old one, the materials of which the foundation is
composed may have decayed so much in process of time as to be
no longer strong enough to carry the superincumbent weight.

Instances such as these may be enumerated by the score, and
the time spent in their investigation can never be considered as
wasted, for they teach us what to guard against in the future ;
and in examining a dangerous structure the knowledge of
defects in other cases often helps us in finding out the reasons
of failure in the case before us.

If a wall whose foundations have thus been discovered to be
at fault is in all other respects in a comparatively sound and
homogeneous condition, i. e. if there are no very serious cracks
or sharp bulges perceptible on its face, or if it is only a few
inches out of the perpendicular, it can be restored to a perfectly
sound and healthy condition by removing the bad foundations
and replacing them, either wholly or in part, with good and
reliable new work. This operation is called underpinning.

It is carried out in the following manner : — Raking shores
are first erected to assist in supporting the wall, and the ground
on either side of it is then dug out at one point only, generally
at the centre ; and it will depend upon the condition of the
brickwork or the masonry of which the wall is composed as to
how many feet along the wall this excavation may extend.
Good brickwork will carry itself over a span of 6 feet or even
7 feet, and the same may also be said of most kinds of
dressed masonry; but when the foundations of a wall have

ON NEEDLE SHORING AND UNDERPINNING 27

failed the homogeneity of the material of which it is composed
is partially destroyed, and it will not be safe to underpin, as a
general rule, more than 3 feet at a time. All the foundation
comprised within this dimension, whether it be brick, stone,
concrete, wood, or iron, is removed entirely, and a new founda-
tion is commenced upon the solid ground, and built up within
the cleared space to the under side of the old work.

Before, however, this new work is commenced the ground on
which it is to be built must be thoroughly examined, and if
necessary inspection shafts should be dug for this purpose ; for
the neglect to examine the ground may have been the original
cause of the failure. After it has been proved satisfactorily
that the ground is fit to be built upon, a good bed of Portland
cement concrete should be carefully laid in a trench cut to
receive it. It should not be allowed to be thrown in from the
ground level as is so often done, for in that case all the larger
stones fall first; but it should be let down in buckets and
quietly deposited, and, after it has been well rammed, the
cement on the top should be flushed off to a level surface. If
brickwork is to be built upon this concrete, slabs of York stone
are often laid over it to receive the footings. A good workman
will measure the distance from the surface of this stone to the
under side of the wall above, and will so arrange his courses
that they will fit into the space exactly, allowing for the
breadth of the joints ; but if, when the work has been carried up,
it is found that the last course does not quite reach to the
under side of the whole work, a carefully laid course of pavement
tiles or slates must be pinned into the space, and well grouted
with liquid cement. The whole of the new work throughout
must be built in cement; for cement possesses the quality
invaluable in this case, of expanding as it sets, and consequently
it causes the whole of the new work to rise slightly and press
against the under side of the old work.*

* (I have allowed this remark of the author's to stand, as it expresses an
opinion shared by not a few experienced architects and builders. Portland
cement, when " hot," expands in setting as we know, but when " cold " and

28 SHORING AND UNDERPINNING.

When this new pier, as we may call it, has been finished,
and the cement has set hard, similar spaces may be cleared
away and new work built and bonded into it on either side; and
so we can proceed until the whole of the old foundations have
been removed and replaced by new work, which will carry the
superstructure with perfect safety for all time to come.

We have stated above that in all underpinning operations
the new work throughout should be built in cement ; this is
certainly correct for all brickwork or masonry, but an exception
may be made to this rule as far as concrete is concerned. All
concretes, whether lime or cement, will expand when they set,
The ordinary lime concrete used in and about London, composed
of six parts of ballast to one part of greystone lime, will expand
as much as three-eighths of an inch to every foot in height, and
the size thus gained the concrete never loses. Consequently, if
the underpinning is all under the ground, lime concrete, which
is infinitely cheaper than brickwork or masonry in cement,
may be the sole material employed; but, and this is important,
some artificial means must be employed to force it up against
the under side of the old work.

A very successful example of underpinning in lime con-
crete only, is thus described by Lieut.-Colonel Sir William
Denison, E.E., in Mr. BurnelTs work on "Limes, Cements, and
Mortars" : —

" One of the large storehouses in Chatham Dockyard having
for some time exhibited serious defects in its walls, the attention
of the Admiralty was directed to it in the year 1834, and
Mr. Taylor, the Civil Engineer and Architect, was directed to
report on the best mode of obviating the evil.

" Upon investigation, the foundation of the storehouse (a

" dead " it shrinks. When sufficiently but not excessively air-slaked it neither
expands nor shrinks, and it is in this condition that it is safest to use cement
for work in underpinning. It requires very great judgment to use expansive
cement, for the expansion may readily be greater than is desirable, and
instances have been known of walls being raised some inches in this manner.
Cement which is " dead " must obviously be carefully avoided.— F. R. F.)

ON NEEDLE SHOEING AND UNDERPINNING. 29

building 540 feet in length and 50 feet in breadth) was found
to be in a very bad state ; the front wall, nearest the river, had
originally been built upon piles, while the rear wall was laid
upon an upper stratum of 5 or 6-inch planking, supported by
two rows of transverse and longitudinal oak sleepers lying on
the surface of the ground, which in this case was of a variable
consistence, containing flints bedded in a sort of clay, quite
pervious to the water, which at high tide rose some height upon
the foundation. The sleepers and heads of the piles at the
front of the building, thus exposed to alternate moisture and
dryness, were in a state of rapid decay : in some places they
were even reduced to a powder, and it was possible for a man
to move under the walls in the space previously occupied by
the timber. In the rear, the case was pretty much the same ;
the sleepers were universally in a state of decay, but in some
places were much further advanced towards decomposition than
in others.

" The state of the storehouse requiring immediate attention,
it wras resolved to attempt to underpin the walls. This the
patentee for the new description of concrete, or artificial stone,
undertook to do, having adopted a plan proposed by Mr. Taylor,
for forcing the soft concrete against the under part of the wall ;
and he proceeded to execute this contract in the following
manner.

" I must premise that the storehouse was vaulted underneath,
and that the piers, or cross walls, required as much underpinning
as any other part of the building.

" The walls were laid open to their bottom, both inside and
outside the building ; in the front, the heads of the piles and
the sleepers were removed for a depth of about 4 feet below
the bottom of the wall, and for lengths of about 5 feet at one
time. In the rear, all the planks and sleepers were removed for
the same distance. A mass of concrete, composed of one-eighth
of Hailing lime (reduced to a powder by grinding, and in a
perfectly caustic state) and seven-eighths of Thames ballast,
mixed up with so much boiling water as to reduce the whole to

30 SHORING AND UNDERPINNING.

a pasty consistence, was then thrown from a height of about
15 feet underneath the wall ; it was allowed to project about a
foot on each side, where it was confined by planks, and after
being roughly levelled, it was well rammed, to give it as much
consistence as possible. This mass was raised about 3 feet, or
to within 1 foot of the bottom of the wall : it was then carefully
levelled, and covered with ^-inch slates. A kind of framework
was then placed on the slates, consisting of two cross-plates of
iron, placed perpendicularly to the direction of the wall, about
1 foot wide, and long enough to project about 1 foot on each
side of the wall.

" To these were fixed two frames parallel to the wall, about
4 feet long, each carrying two sockets for screws. Within these
frames were placed two movable planks, long enough to pass
just free between the cross-plates, and wide enough to fit nearly
the space between the slates and the bottom of the wall. Upon
these planks were sockets for the heads of the two screws,
by which the planks were pushed forward or withdrawn at
pleasure.

" When the apparatus was fixed, and the movable planks
ready on both sides of the wall, about two barrowfuls of con-
crete, mixed as stated, were thrown in from above ; the work-
men below then commenced turning the screws on each side
simultaneously, moving the two planks towards the centre of
the wall, and forcing the concrete before them into all the
vacant spaces, and against the bottom of the wall. When the
plank was forced forward as far as it would go, by the strength
of two men to each screw, the concrete was allowed to rest for
about five or ten minutes, by which time it had set hard enough
to stand by itself, and its expansion in the act of setting com-
pleted what the pressure of the screws might have left undone.
The planks were then withdrawn, another charge thrown in on
each side, and compressed as before, and this was continued
till the whole space between the frames was filled with concrete.
The screws were then removed, the boards and frames unbolted
and taken out, and lastly, the side-plates were withdrawn,

PLATE, VI.

J Alurmtri Phct»-htVI,ol,aon,W C

ON NEEDLE SHORING AND UNDERPINNING. 81

leaving an interval of about •§ of an inch between each mass of
concrete, which space was afterwards filled in with grout.

" The above description is given from notes taken at the
time. The proportion of lime to gravel is as 1 to 6 ; and such
is the efficiency of the concrete in the mode in which it was
applied, that no settlement has taken place since the work was
completed."

The majority of underpinning operations are carried out by
some such methods as these that have now been described ; but
this way of dealing with a ruinous structure may be considered
rather in the light of a prevention than a cure, for unless a
building is thus treated at once when its foundations first show
signs of giving way, the evil will gradually increase, and
render it imperative not only that the foundations should be
renewed, but also that a considerable portion of the wall above
the ground should be taken down and rebuilt.

If we look, for instance, at the wall of the house depicted on
Plate VI. Figs. 1 and 2, we shall see that it is ruined for several
feet above the foundations. This might, perhaps, have been
prevented if it had been underpinned at once, when the failure
first showed itself ; but no such steps having been taken, it has
cracked badly in many places, bulged forward, and dragged the
return walls out of the perpendicular.

The reason why the foundations have so signally failed in
this case to carry the superstructure, is because the house has
been made to encroach upon the site of an old pit or trench,
shown by the dotted line in the section, Fig. 2, which has been
filled up for many years, so that its existence has perhaps never
been suspected : and as the foundations do not go down to any
great depth, it is quite possible that it may not have been
noticed when the wall was built, or the contractor may have
chosen rather to risk a settlement than go to the extra expense
of excavating the made ground and building up from the virgin
soil.

It is now too late to underpin this wall in the ordinary way
that we have just described, for the evil has spread so far that

32 SHORING AND UNDERPINNING.

it would still be unsafe, even though its foundations were
renewed ; but after it has been well shored up with raking
shores, the method of needle shoring must be employed to
support the upper portion (which, though it has been squeezed
in a little towards the centre, is otherwise comparatively sound
and homogeneous), and the whole of the lower portion of the
wall for a distance of about 12 feet from the ground must be
removed entirely. Accordingly four needles, which it is best to
make whole timbers, i. e. about 13 inches by 13 inches, are
inserted through holes cut in the wall well out of reach of the
cracks, and above the point where the bulge is most pronounced,
and these are supported by eight posts of the same scantling.
In consequence of the peculiar nature of the case, the posts
must be placed upon two continuous sole-pieces, laid on either
side of the wall, and stretching well across the treacherous
ground. On the exterior of the wall, the sole-piece must rest
upon a carefully laid platform of stout planks, laid in such a
way that the bearing of the two central posts may be spread
over so much of the surface that it will be impossible for them
to sink when the weight comes upon them. On the inside, the
sole-piece may rest upon the concrete under the floor, if it is in
a good condition ; but if not, it must rest upon a similar plat-
form of timber to that on the outside of the building. Great
care must be taken in arranging these platforms that there may
be no possibility of their being disturbed when the ground is
excavated for the new foundations.

It will be noticed in the section, Fig. 2, that the needles pass
through the wall just above one of the floors. This is the best
and most usual position for the needles to occupy; for the brick-
work at this point, and for some way above it, is perfectly
sound, and has not been cut into for the insertion of plates and
joists. This floor must, of course, be strutted up independently
of the wall, and a hole cut through it and the ground floor to
allow the posts which carry the needles to pass freely to the
ground. If, however, this cutting through of the floors would
be a very costly and troublesome business, platforms of timber

ON NEEDLE SHORING AND UNDERPINNING. 33

may be placed upon the floor and on the under side of the
ceiling, spreading the weight over as many joists as possible,
and the posts set up in different pieces.

The needles having been wedged up tightly against the under
side of the old work, the whole of the brickwork below the
needles must be taken down, the made ground under the wall
dug out, and a good trench cut in the virgin soil to receive the
bed of new concrete. The rest of the work may then be built
up again in cement to meet the wall above, in the same way as
has been described already.

It should always be borne in mind, that even after this new
work has been finished and the cement has set, it is still the
needles and posts which do the real work of carrying the
wall; and the greatest care must be exercised in removing
them, that the weight is transferred gradually, and not all
at once, upon the new work. The needles should first be
eased a little by knocking out the wedges at the foot of the
posts a few inches, and then, after a day or two has elapsed,
the wedges may be withdrawn entirely and the needles taken
out; but the raking shores should remain in position for
about a week after the wall has settled down upon its new
bearings.

With regard to the responsibilities incurred in case of the
failure of underpinning operations, Mr. Edwin Nash, in his
paper on "Failures in Construction,"* makes the following
remarks : — " When we see that accidents under this head may
cause verdicts of manslaughter to be recorded against architects,
as was the case against Mr. Abraham, after the noted fall of a
house in the Strand in 1853, we must be awakened to the
necessity of so arranging the business part of such operations
that the architect shall not be made responsible for details he
cannot control. It is often a sort of work that requires intelli-
gent watching during every moment of its progress, and this is
not the architect's business ; and if this view be not recognised

* " Transactions R.I.B.A.," vol. xiv.
S. D

34 SHORING AND UNDERPINNING.

by courts of law, it behoves us to define the responsibility in
a written document between architect and builder before
commencing the work."

It is not, however, in connection with ruinous structures that
we must look for the most general use of needle shoring, for
walls are, as a rule, underpinned at once, without its aid, when
the foundations first show signs of giving way ; but it is much
more commonly employed in cases where some alteration is to
be made in a building which is perfectly sound — such, for
instance, as the addition of a new basement, or the insertion of
a shop-front. As an example of the former case, the Gaiety
Eestaurant in the Strand may be cited. It was necessary, in
order to obtain the space afterwards occupied by the magnificent
Grill Eoom, that the walls should be taken down to a greater
depth than was previously the case ; accordingly needle shoring
was employed, and the whole building stood for many weeks as
it were upon crutches, while the new foundations were being
built. In consequence of the weight of the walls, and to
obviate some difficulty in supporting the floors, the needles
were doubled, i.e. placed one above another in the manner
shown in the sketch, Plate VI. Fig. 5.

An example where needle shoring is required to support a
wall during the insertion of a breastsumrner and shop-front is
illustrated in Plate VI. Figs. 3 and 4. The needles in this case
must be made longer than usual to span the vaults under the
pavement; consequently it will be as well to strut them as
shown in the section, Fig. 4. Eaking shores need not be used
unless the wall is of a great height, or in a bad condition ; but
the window openings immediately above the needles, must in
any case be well strutted, as shown in the elevation, Fig. 3, or
they will be squeezed in, and the frames, as well as the glass,
will be broken.

When the opening has been made in the wall, and substantial
piers have been built at either end of it, the girder or breast-
summer is fixed in position, and a plate fitted to its lower flange
to take the joists of the first floor. A 3 -inch York stone template

ON NEEDLE SHORING AND UNDERPINNING. 35

is then bedded on the top of the girder, and a course or two of
brickwork built up in cement to meet the under side of the old
work.

It will now be unnecessary to give any further illustrated
examples of this simple method of needle shoring : but before
we go on to consider its use in cases of a more complicated
character in the next chapter, there are one or two further
points to which it may be as well to draw the reader's
attention.

If it should be required, for instance, to clear away the lower
portion of a party-wall, so as to throw the premises on the
ground floor into one large shop or office, before the needles,
which will carry the wall during this operation, are inserted,
the following points should be considered : —

1st. If there are any chimney breasts in the wall, they should
be well supported ; two or even three needles, if the width of
the breast is considerable, should be inserted under them, with
as good a bearing as possible.

2nd. If there are any piers or corbels in the wall, a needle
should be inserted under each.

3rd. If the upper floors are double, or framed floors, the
needles should be inserted in the same perpendicular plane
with the binding joists or girders.

4th. Care should be taken in arranging the position of the
posts which are to carry the needles, that they shall not inter-
fere with the proper adjustment of the girders and stanchions
which are eventually to carry the wall.

The needles must be inserted above the first floor for the
reasons mentioned above, and also to allow of the girders
being fixed on a level with this floor. In the case of a ware-
house, if the structure is in a bad condition, it will be as
well to remove all goods which are stored upon the floors
above the needles, or at all events to shift them, so that their
weight is carried by the story posts or by another wall. But
if this could not be done except at great inconvenience to
the proprietors, the floors must be strutted up from the

D2

86 SHORING AND UNDERPINNING.

ground, and so made altogether independent of the support of
the party-wall.

In the example we have given above of the underpinning of
the storehouses at Chatham, it was not deemed necessary to
move the goods at all during the operations, though they were
very heavy, comprising all sorts of ships' tackling, such as
cables, blocks, ropes, spars, &c. ; but it will be recollected that
in that case the walls were not needled, and only underpinned
in short lengths at a time. However, all such considera-
tions as these depend upon so many things, that they can
only be left to the judgment of the architect in each particular
case.

At the commencement of this chapter it was laid down as a
general rule that whole timbers should be used in cases of
needle shoring, and for this reason, that although the scantling
of whole timber may be found by the usual formula to be larger
than that required to carry the weight with safety, yet it should
be borne in mind that all beams of timber will deflect a little
when a weight comes upon them, and ii is important iu the case
of a needle that this deflection should be reduced to a minimum.
Again, there is always the possibility of there being some
unforeseen defect in the timber, and the greatest care should
be taken, even when a needle is made of whole timber, that it is
perfectly sound throughout : the same may also be said of the
posts which carry the needles. If, however, economy or space
should require that smaller timbers should be used, we must
employ the following formulae, from " Tredgold's Carpentry,"
for the scantling of beams supported at both ends and loaded
in the centre, and posts compressed in the direction of their
axis : —

To find the scantling for a rectangular piece of timber that
will sustain a given weight in the centre, when supported at
the ends in a horizontal position.

When the breadth is known or settled,

ON NEEDLE SHORING AND UNDERPINNING. 37

When the depth is known or settled,
I/'xWxa.

where L = length of bearing in feet ;

W = weight to be carried in pounds ,'
a = -01 for fir, and -013 for oak ;
B = breadth in inches ; and
D = depth in inches ;

To find the scantling of a rectangular post capable of
sustaining a given pressure in the direction of its length :

where L = length in feet ;

W = the weight to be sustained in pounds ;
a = 0-00133 for fir, and 0-0015 for oak ;
B = breadth in inches ; and
D = thickness required in inches.

Part, IX. of the London Building Act, 1894 (57 & 58 Viet,
c. 213,), relating to Dangerous and Neglected Structures.

Dangerous Structures.

Sect. Oil. In this part of this Act the expression "structure" Meaning of
includes any building, wall, or other structure, and anything
affixed to or projecting from any building, wall, or other
structure.

Sect. GUI. — (1) Where it is made known to the Council Survey to
that any structure is in a dangerous state the Council shall dangerous
require a survey of such structure to be made by the district
surveyor or by some other competent surveyor.

(2) For the purposes of this part of this Act the expression
" district surveyor " shall be deemed to include any surveyor so
appointed.

(3) The district surveyor shall make known to the Council

88

SHORING AND UNDERPINNING.

Effect of
this part of
Act within
the City.

Surveyor
to give
certificate.

Notice to
be given to
owner in
respect of
certificate.

Proceed-
ings to
enforce
compliance
with
notice.

any information which he may receive with respect to any
structure being in a dangerous state.

(4) It shall be lawful for the district surveyor to enter into
any structure or upon any land upon which any structure is
situate for the purpose of making a survey of such structure.

Sect. CIV. In cases where any such structure is situate
within the city this part of this Act relating to dangerous
structures shall be read as if the Commissioners of Sewers
were named therein instead of the Council, and all costs and
expenses of and all payments hereby directed to be made by or
to such Commissioners shall be made by or to the Chamberlain
of the City out of or to the consolidated rate made by such
Commissioners in the same manner as payments are made by
or to such Chamberlain in the ordinary course of his business.

Sect. CV. Upon the completion of his survey the district
surveyor employed shall certify to the Council his opinion as to
the state of the structure.

Sect. CVI. If the certificate is to the effect that the structure
is not in a dangerous state no further proceedings shall be had
in respect thereof, but if it is to the effect that the same is in a
dangerous state the Council may cause the same to be shored
up or otherwise secured, and a proper hoard or fence to be put
up for the protection of passengers, and shall cause notice to be
served on the owner or occupier of the structure requiring him
forthwith to take down, secure, or repair the same as the case
requires.

Sect. CVII. — (1) If the owner or occupier on whom the
notice is served fail to comply as speedily as the nature of the
case permits with the notice, a petty sessional court on com-
plaint by the Council may order the owner to take down, repair,
or otherwise secure to the satisfaction of the district surveyor
the structure or such part thereof as appears to the court to be
in a dangerous state within a time to be fixed by the order, and
if the same be not taken down, repaired, or otherwise secured
within the time so limited, the Council may with all convenient
speed cause all, or so much of the structure as is in a dangerous

ON NEEDLE SHORING AND UNDERPINNING. 39

condition to be taken down, repaired, or otherwise secured in
such manner as may be requisite. Provided that if the owner
of the structure dispute the necessity of any of the requisitions
comprised in the notice, he may by notice in writing to the
Council within seven days from the service of the notice upon
himself, require that the subject shall be referred to arbitration.

(2) In case the owner require arbitration, he may at the
time of giving such notice appoint an independent surveyor to
report on the condition of the structure in conjunction with the
district surveyor within seven days of the receipt by the Council
of the notice of appointment of the owner's surveyor, and all
questions of fact or matters in dispute which cannot be agreed
between the owner's surveyor and the district surveyor shall
be referred for final decision to a third surveyor, who shall
(before the owner's surveyor and the district surveyor enter
upon the discussion of the question in dispute) have been
appointed to act as arbitrator by such two surveyors, or in the
event of their disagreeing by a petty sessional court on the
application of either of them. Such arbitrator shall make his
award within fourteen days.

(3) The notice served by the Council shall be discharged,
amended, or confirmed, in accordance with the decision of the
two surveyors or the arbitrator as the case may be.

(4) Unless the arbitrator otherwise direct the costs of and
incident to the determination by the two surveyors or the
arbitrator of the question in dispute shall be borne and paid in
the event of such determination being adverse to the contention
of the district surveyor by the Council, or in the event of such
determination being adverse to the contention of the owner's
surveyor by the owner.

Sect. CVIII. Notwithstanding any such notice requiring Conrtmay
arbitration as aforesaid a petty sessional court, on complaint by notwith- e
the Council, may, if of opinion that the structure is in such a arbitral
dangerous condition as to require immediate treatment, make
any order which such court may think fit with respect to the
taking down, repairing, or otherwise securing the structure.

40

SHORING AND UNDERPINNING.

Exper

Provisions
respecting
sale of
dangerous
structures.

If proceeds
insuffi-
cient, land
not to be
built on
till balance
paid

Recovery of
expenses.

Fees to
surveyor.

Sect. CIX. — (1) All expenses incurred by the Council in
relation to the obtaining of any order as to a dangerous struc-
ture, and carrying the same into effect under this part of this
Act, shall be paid by the owner of the structure, but without
prejudice to his right to recover the same from any person
liable to the expenses of repairs.

(2) If the owner cannot be found, or if on demand he refuse
or neglect to pay the said expenses, the Council after serving
on him three months' notice of their intention to do so may,
if in their discretion they think fit, sell the structure, but they
shall, after deducting from the proceeds of the sale the amount
of all expenses incurred by them, pay the surplus (if any) to the
owner on demand.

Sect. CX. Where under this part of this Act any dangerous
structure is sold for payment of the expenses incurred in
respect thereof by the Council, the purchaser, his agents and
servants may enter upon the land whereon the structure is
standing for the purpose of taking down the same and of
removing the materials of which it is constructed.

Sect. CXI. Where the proceeds of the sale of any such
structure are insufficient to repay to the Council the amount of
the expenses incurred by them in respect of such structure, no
part of the land whereon the structure stands or stood shall be
built upon until after the balance due to the Council in respect
of the structure has been paid.

Sect. CXII. If the materials are not sold by the Council, or
if the proceeds of the sale are insufficient to defray the said
expenses, the Council may recover the expenses or the balance
thereof from the owner of the building, together with all costs
in respect thereof in a summary manner.

Sect. CXHL— (1) There shall be paid to the district surveyor
in respect of his services under this part of this Act in relation
to any dangerous structures the fees specified in Part II. of the
Third Schedule to this Act.

(2) Provided that if any special service is required to be
performed by the district surveyor under this part of this Act

ON NEEDLE SHORING AND UNDERPINNING. 41

for which no fee is specified in the said schedule, the Council
may order such fee to be paid for that service as they think fit.

(3) All fees paid to any surveyor by virtue of this section
shall be deemed to be expenses incurred by the Council in the
matter of the dangerous structure in respect of which such
fees are paid, and shall be recoverable by them from the owner
accordingly.

Sect. CXIV. Where a structure has been certified by a Power to
district surveyor to be dangerous to its inmates, a petty ses- inmates
sional court may, if satisfied of the correctness of the certificate, dangerous
upon the application of the Council, by order direct that any
inmates of such structure be removed therefrom by a constable
or other peace officer, and if they have no other abode he may
require that they be received into the workhouse for the place
in which the structure is situate.

Neglected Structures.

Sect. CXV. — (1) Where a structure is ruinous or so far Removal of
dilapidated as thereby to have become and to be unfit for use 6

or occupation, or is from neglect or otherwise in a structural
condition prejudicial to the property in or the inhabitants of
the neighbourhood, a petty sessional court on complaint by the
Council may order the owner to take down or repair or rebuild
such structure (in this Act referred to as a neglected structure)
or any part thereof, or to fence in the ground upon which it
stands or any part thereof, or otherwise to put the same or any
part thereof into a state of repair and good condition to the
satisfaction of the Council within a reasonable time to be fixed
by the order, and may also make an order for the costs incurred
up to the time of the hearing.

(2) If the order is not obeyed the Council may, with all
convenient speed, enter upon the neglected structure of such
ground as aforesaid and execute the order.

(3) Where the order directs the taking down of a neglected
structure or any part thereof, the Council in executing the order

42 SHORING AND UNDERPINNING.

may remove the materials to a convenient place, and (unless
the expenses of the Council under this section in relation to
such structure are paid to them within fourteen days after such
removal) sell the same it and as they, in their discretion, think
fit.

(4) All expenses incurred by the Council under this section in
relation to a neglected structure may be deducted by the Council
out of the proceeds of the sale, and the surplus (if any) shall be
paid by the Council on demand to the owner of the structure,
and if such neglected structure or some part thereof is not taken
down and such materials are not sold by the Council, or if the
proceeds of the sale are insufficient to defray the said expenses
the Council may recover such expenses or such insufficiency
from the owner of the structure together with all costs in respect
thereof in a summary manner, but without prejudice to his
right to recover the same from any lessee or other person liable
to the expenses of repairs.

Supplemental as to Dangerous and Neglected Structures.

Sect. CXVI. — (1) Where the Council have incurred any
expenses in respect of any dangerous or neglected structure,
and have not been paid or have not recovered the same, a petty
sessional court on complaint by the Council may make an order
fixing the amount of such expenses and the costs of the
proceedings before such petty sessional court, and directing
that no part of the land upon which such dangerous or neglected
structure stands, or stood, shall be built upon, or that no part
of such dangerous or neglected structure, if repaired or rebuilt,
shall be let for occupation until after payment to the Council of
the said amount, and thereupon and until payment to the
Council of the said amount no part of such land shall be built
upon, and no part of such dangerous or neglected structure so
repaired or rebuilt shall be let for occupation.

(2) Every such order shall be made in duplicate, and one
copy of such order shall be retained by the proper officer of the
court and the other copy shall be kept at the county hall,

ON NEEDLE SHORING AND UNDERPINNING. 43

(3) The Council shall keep at the county hall a register of all
orders made under this section, and shall keep the same open
for inspection by all persons at all reasonable times, and any
such order not entered in such register within ten days after
the making thereof shall cease to be of any force. No property
shall be affected by any such order unless and until such order
is entered in such register.

Sect. CXVIL— The fees specified in Part IV. of the Third
Schedule to this Act as payable to the Council, shall be payable or neg-
to and may be recovered in a summary way by the Council. structures

J J J J to Council.

CHAPTEB VI.

ON THE SHOEING AND UNDEEPINNING OF
MEDIAEVAL BUILDINGS.

THE practice of the art of shoring and underpinning does not
always confine itself to the meaner buildings in a crowded
town, but the sphere of its greatest usefulness and fullest
development is to be found in the restoration of our venerable
churches and cathedrals, many of which, but for its timely aid,
would long before this have fallen victims to the ravages of
decay. Every architect who loves his art must be glad to be
the means of saving from destruction even one stone of those
wonderful and beautiful works executed by the masons of the
Middle Ages : and there have been many instances in which
the ponderous towers and steeples of cathedrals have been saved
from impending ruin by an opportune application of this useful
science. Such were the works of Eondelet at the Church of St.
Genevieve at Paris, of Flachat at the cathedral at Bayeux, and
in our own country, of Cottingham at Hereford ; and had it not
been for the interference of the elements, the underpinning at
Chichester would no doubt have been successfully carried out,
and the original tower and spire of the cathedral might still
have been standing.

The gigantic shores and centres used in cases such as these
require, however, a fuller description than can be given in this
treatise ; and the reader is referred for an example to the
excellent description and drawings of the shoring of the central
tower and lantern of the cathedral at Bayeux by MM. Dion
and Lasvignes. But instances of shoring on so vast a scale
are rare, and more the work of engineers than architects ; and
it will be better to describe here a more humble example,

ON THE SHORING OF MEDIAEVAL BUILDINGS. 45

and one which is more likely to be of service to us in ordinary
practice.

At a meeting of the Eoyal Institute of British Architects,
held on Monday, 3rd February, 1873, an excellent paper was
read by Mr. J. P. Seddon, F.R.I.B.A., on the shoring, &c., of the
tower and spire of the parish church of Grosmont in
Monmouthshire. We cannot do better than quote here Mr.
Seddon' s own remarks upon that building, describing the state
of decay in which he found it, and the subsequent measures
which were employed in its restoration. The diagrams on
Plates VII. and VIII. are copied from the drawings made by
Mr. William Ed. Martin to illustrate Mr. Seddon's paper, and
which afterwards appeared in the Building News of February
7th, 1873.

" The parish church of Grosmont, dedicated to St. Nicholas,
in the diocese of Llandaff, is situated in Monmouthshire, near
to where the border of that county joins those of Herefordshire
and Breconshire — a very beautiful and retired part of the
country.

" The structure is one which by its historical interest and
architectural value justifies the pride taken in it by the
inhabitants of the surrounding district ; but it has even wider
claims for consideration, and particularly in connection with
this metropolis, distant though it may seem to be.

" It owes, if not its origin, at least its enlargement and
embellishment, to the same munificent patronage which directed
those on a grander scale at the Abbey of Westminster ; and
though Grosmont Church is, as befits its position, a compara-
tively humble structure in point of style, it may claim some
resemblance to its nobler contemporary. Had the same caution
been exercised in its case as in that of the Abbey, and had only
a modest lantern surmounted its crux, I should not have the
following chronicle of disaster to bring before you. But the
substructure was in all probability never intended to support
the ambitious though elegant central octagon tower and spire
which at a later period were piled upon it, exemplifying a

46 SHORING AND UNDERPINNING.

temerity of which mediaeval architects were often guilty, and
which brought ruin in the case of Chichester and serious danger
in that of Salisbury.

" The church, the plan of which is a Latin cross, consists of
a nave 67 feet by 18 feet 6 inches, and aisles 9 feet 6 inches
wide, separated by arcades of five bays (with responds deeper
than ordinary, obviously to give more abutment to the crux
arches) ; central tower and spire ; transepts with aisles on the
western sides of the same width as those to nave ; chancel and
chapel south of same. There is also a porch on the north side
opposite the central bay of the main arcades. The crux arches
and transept are the earliest portion, being in the style of the
Transition between Norman and Lancet. The chancel is fully
developed Lancet.

" It is now many years since I was first called in to examine
this church, and then it was in a condition which cannot be
described as other than tottering from old age. In this part of
the country it must always have been a difficulty to obtain
proper building sand, and the loamy sand at hand would soon
destroy the value of any amount of lime mixed with it. From
this cause the mortar of the walling throughout had become
little better than earth, and the whole of the external walls
exposed to the weather were grievously dilapidated.

" Under the great weight of the tower and spire which were
added, the earlier crux, piers, and arches have been crushed and
twisted out of shape, and this pressure has been transmitted in
the directions of north, south, and west, by the several arches,
which had themselves become distorted so as actually to thrust
outwards the end walls of nave and transepts. The more solid
walls of the eastern side of transepts and of the chancel had
yielded less, yet still in some degree.

" The whole eastern limb, viz. chancel and Eleanor Chapel,
by far the richest architecturally, was in the worse condition,
and imperatively needed rebuilding. Under the circumstances
described, however, it seemed a perilous operation to undertake,
as even the temporary removal of such support as they gave the

ON THE SHORING OF MEDIAEVAL BUILDINGS. 47

central tower might accelerate the ruin of the rest of the fabric.
Funds adequate for this work only having with difficulty been
collected, this was effected with great care. The chancel and
Eleanor Chapel were in 1869-70 almost entirely taken down
and rebuilt under my directions.

" Careful examination was made, before and after the execu-
tion of this work, of the state of the crux, piers, and arches, and
marks set to show whether these yielded at all by reason of
settlement in the new masonry. This, which was mostly to be
feared at the north-east angle pier, does not seem to have taken
place to any great extent. But still I received reports from
time to time that the original . mischief was proceeding, and I
caused a close examination to be made, from which it appeared
that the cracks were surely though slowly extending, particularly
in the north-west pier. In consequence of this, I reported that
it was, in my opinion, essentially necessary that the tower and
spire should be so shored up and supported by centres as to be
independent of the piers, which then, as funds were procured,
could be made good ; after which the restoration of the arches
and superstructure could at any time be taken in hand.

" I estimated the cost of this preliminary work of supporting
the failing arches at about 400Z., and received instructions from
the vicar, the Bev. W. H. Twyning, to direct it to be done at
once.

" The failure of the substructure of the tower is primarily
traceable to two causes. First, errors in design ; and, secondly,
errors in construction. The design is in fault from the weight
of the tower being carried upon insufficiently abutted arches ;
and the construction, from the imperfect execution of the
dressed stone-work and the masonry of the walling.

" From the first cause (imperfect design) four distinct classes
of failure are to be traced : — (1) Spreading of arches at their
springing ; (2) flattening of the arch curves, thus neutralising
the keying, and rendering the arch insecure by the liability of
voussoirs to fall out ; (3) thrusting of the vertical supporting
piers under the tower arches out of the perpendicular; and

48 SHORING AND UNDERPINNING.

(4) transmission of the thrusting force to all adjoining piers,
arches, and walls, throwing them out of the normal stable
condition — verticality.

" From the second cause (imperfect construction) three classes
of failure may be traced : — (1) The crushing of the wrought
stone facings which form the casing of the piers ; (2) the
bursting asunder or drawing of the bonders of the various
members of which the piers are composed; and (3) rents or
fissures of the walling generally.

" The most prolific causes of failure in building are generally
two, viz. unequally yielding of foundation trenches, and un-
compensated thrusts, whether from roofs or arches. The case
now under consideration is a signal example of failure from
the latter cause — an equally unyielding foundation having
contributed in some degree to intensify this failure.

" Writers of books on building generally assume it as a fact
not to be questioned that a solid rock foundation, roughly
levelled or stepped where necessary, is the foundation most to
be desired ; but an attentive consideration of the present case
would lead to the belief that such a foundation, if not absolutely
dangerous as a base for a building erected in the ordinary way,
is at least very undesirable unless extraordinary precautions are
used in the selection of the materials for the walls, in the bond-
ing, and in the elimination of all unequal settlement from a
greater number of mortar joints in any one portion of the
walling than in another on the same level. In this case the
functions of the tower piers were to transmit the weight of
the tower to the foundations ; the latter being rock and incom-
pressible, the piers became crushed between two unyielding
forces, which would not have been the case had the foundation
been of a partially yielding nature, such as a stiff clay or
gravel.

" Taking the various classes of failure enumerated in detail :
(1) The spreading of the lower arches at the springing. The
four arches carrying the tower spread as follows : — North arch,
• 584 feet (7 inches) ; east arch, • 375 feet (4£ inches) ; south arch,

ON THE SHORING OF MEDIEVAL BUILDINGS. 49

• 75 feet (9 inches) ; west arch, • 625 feet (7£ inches). This
spreading has not taken place equally at both sides of the
original central line of each arch; the abutments to some of
the arches, being more solid and stable than others, remain
almost in their original positions, whilst the spreading has
taken place on that side of the centre line towards the weakest
abutment.

" Spreading of the arches leads naturally to the second class
of failure, viz. flattening of the arch curves. This flattening
has not taken place regularly ; the arches preserve in some parts
their original curves, whilst in other places the curves have
been forced into straight lines. The general outlines now
assumed by the soffits of the arches are irregular lines not
amenable to any known mathematical curve.

"Spreading of the arches also involves the third class of
failure, viz. thrusting the piers supporting them out of the
perpendicular. It is evident that the piers could not have
remained upright when the arches spread, except on the sup-
position that the springers of the arches slipped back on the
abaci of the caps ; but this would have been impossible, for the
vast weight of the superstructure augmented the friction between
the two stone surfaces to such an extent as to make the last
stone of the cap and first stone of the arch practically one
stone. Hence the number of inches by which the faces of two
opposite piers are out of plumb becomes a correct measure of
the spread of the superincumbent arch.

" The fourth class of failure noticed is the transmission of the
thrusts of the tower arches to the extremities of the building
in all directions. It will be well to remember that those forces
commenced and continued to act whilst the walling generally
was green and the mortar in a soft condition, thus facilitating
to some extent the accommodation of the surrounding abutments
to the thrusting forces, without involving any sudden violent or
dangerous fractures ; while the gradual subsequent piling on
weight when the tower and spire were added, continued to
increase the distortion.

50 SHORING AND UNDERPINNING.

" The forces generated by the thrusting of the north and
south tower arches are in the directions of the nave arcades to
the westward and the chancel flank walls to the eastward;
the latter, being comparatively solid walls — on account of the
narrowness of the lancet window openings — have sustained the
thrusts in a fairly efficient manner ; but on account of the large
openings and small piers in the nave arcades they formed but
an indifferent abutment ; hence every pier and arch is thrust
westward, the west gable itself being thrust out of the per-
pendicular, overhanging its base 5f inches. The east and west
tower arches, acting through the transept flank walls which
are their abutments, have thrust out of the perpendicular the
north and south transept and walls — the former 4f inches, and
the latter 8£ inches.

" An inspection of the ground plan of the building will
show the north-west and south-west piers to be those most
deficient in abutment, and in reality it is found that these two
piers are those that have suffered most, and are in the most
dangerous condition. The south-west pier had to be cased
some forty years since with carefully coursed wrought masonry,
increasing the area of the pier by about 10 feet superficial ; and
the present extremely dangerous condition of the north-west
pier compels its reconstruction before any other portion of the
building.

" The first class of failure arising from the second cause is
the crushing of the dressed stonework in the pier facings.
This has taken place from the undue concentration of the
weight on this facing ; the backing being composed of rubble
walling, with a greater number of mortar joints than in the
facing, has settled down, leaving the casing to do the work of
carrying the tower, and thus reducing the working area of each
pier from 18 feet to 8-34 feet.

" The second class of failure under this head is the drawing
of the bond stones or bursting asunder of the piers. This
is a very unusual mode of failure ; and is due in this case to
imperfect footings under some members composing the piers.

ON THE SHORING OF MEDIEVAL BUILDINGS. 51

The footings were crushed or squeezed away from this par-
ticular part of the foundations ; hence the bursting or drawing
of the bonders or headers in the quoins immediately over this
defective work.

"The last class of failure to be noted is that most commonly
found in nearly every building, ancient or modern, viz. splitting
of the walling in a direction at right angles or inclined to the
beds, commonly called settlements.

" Settlements result from the non-elastic nature of the
materials composing a wall ; no one part of the walling being
free to sink, or settle down, or change its position, vertically or
horizontally, without fracturing or splitting the stones, bricks,
or mortar joints in a greater or less degree ; always in propor-
tion to the depth of settlement. From the description already
given of the movements of the arches and piers, with their
abutments, it will be no matter of surprise to find the masonry
of the walls generally, in contact with the tower, fractured,
and thrust and crushed in every direction, horizontally as well
as vertically. The entire subject affords an interesting and
instructive example of the effect produced by a weight of
600 tons acting upon four pointed arches for a space of 500
years, and serves to demonstrate conclusively the necessity of
neutralising thrusts effectively, whether such thrusts be created
by the exigencies of style or design.

"The state of the tower, piers, and arches, was, as may be
imagined, the subject of much talk in the village of Grosmont.
The oldest inhabitant recollected the structure to have been in
exactly the same state ever since he first saw it, and by some
extraordinarily subtle process of reason deduced this valuable
conclusion, viz. that as the tower had never fallen in his
time, it was never going to fall. Almost every village in this
part of the world contains at least half-a-dozen of such old
inhabitants, whose inexorable logical deductions are supposed
to silence most effectually the objections of any unfortunate
professional man who happens to disagree with them.

" It having been decided in the autumn of the year 1869 to

E 2

52 SHORING AND UNDERPINNING.

restore the chancel of Grosmont Church, the opportunity of
seeking to determine if the failure of the tower substructure
was at all progressive was seized. With this object all the
fissures in the stonework were filled with cement, and the
extent of the fissures lineally determined by drawing lines
across the end of them in transverse directions. The structure
thus prepared was left, after the chancel had been rebuilt, up
to the end of November, 1872 (about two years), when a careful
inspection of the parts so prepared revealed the following
startling facts : first, that all the fissures which had been
sealed up with cement were open again ; and, secondly, that
the transverse terminal lines of the fissures of 1870 were left
2 inches or 3 inches, in some cases as much as 6 inches, behind
by the extension of the fissures up to 1872. This discovery
compelled immediate attention to the dangerous condition of
the tower, and notwithstanding the renewed protests of the
oldest inhabitants, I did not hesitate to recommend the taking
of immediate steps to restore the four disabled tower piers and
arches, and in the event of the necessary funds not being
available to effect this restoration, at least to shore up three of
the arches, thus relieving the piers, and to needle the fourth
arch, leaving a clear space under it for its restoration should
the funds obtainable be sufficient to cover the expense.

" An idea suggested itself that the piers and arches might be
restored by taking out a damaged stone here and there, and
replacing the stones so removed with other sound stones, thus
effecting the restoration with comparative safety and by slow
degrees ; but on consideration this plan was abandoned, because
some parts of the piers should of necessity be entirely recased
or rebuilt, of course vertically. This would have the effect
of reducing the width between the piers to something about
9 inches less than the width of the arch at the springing,
which would be a reversal of the proper way of treating the
arches, viz. by having them, as originally constructed, 2 inches
narrower at the springing than the space between the piers
supporting them. It was therefore decided that the piers and

ON THE SHORING OF MEDIAEVAL BUILDINGS. 53

arches should be entirely removed and rebuilt, using in all
the old stone not damaged, and that this should be first tried
upon the arch and piers on the north side, the arch proposed
to be needled, this being in the most unsafe condition of the
four.

"As in constructing an effective system of supports to the
tower arches, a safe unyielding bottom was a primary con-
sideration, it was determined in this case to clear away the
entire space immediately under the tower, tower arches, and
for a space of 3 feet all round outside or beyond the tower
piers, right down to the solid rock, and to refill the space so
cleared with carefully made cement concrete well rammed.
The site to be thus operated upon was encumbered with old
seats, fittings, and wood floors, all of which having been cleared
away, the excavation commenced, planked runs having been laid
down through the church and across the churchyard to pits or
graves dug to receive the human remains disinterred ; the soil
itself being spread over the surface of the churchyard at some
distance from the building.

" On removing the soil immediately under the floors it was
found that the bodies had been at some time interred with not
more than four inches of soil over the coffins, which accounted
for a hitherto ' unaccountable smell ' that had frequently
sickened some members of the congregation during their attend-
ance at Divine Service.

" Lower down, at about two feet under the floor level, five
distinct springs made their appearance, evidently the drainage
from the hill at the north side of the building. These springs
flooded the space already excavated, preventing further pro-
gress. A drain six feet deep was cut through the south
transept and discharged through the south transept wall into
the churchyard, which is lower at that side. This drain kept
the working from being submerged, and discharged during the
heavy rains 60 gallons of water per minute.

" The excavations were continued until solid rock was reached
at an average depth of five feet from the floor level. The entire

54 SHORING AND UNDERPINNING.

soil removed was of a very dark colour, light in weight and
spongy in texture, containing human remains in various stages
of decay ; in fact the whole mass had apparently been used over
and over again for burials, the most recent having been appar-
ently thirty-one years ago. This appeared from the coffin
breastplate, which with its gilded lettering, was as fresh as
the day it was put in, although there was no trace whatever of
the coffin, which was stated to have been of oak by a party who
recollected seeing it lowered into the grave.

" Some graves were hollowed out of the solid rock below the
tower foundations, others with steined half brick sides, covered
with stone slabs ; the latter were found to be filled with a
black fluid, emitting a stench so horrible as to be perceived
even in the most remote parts of the building. All human
remains disturbed were reverently cared for and interred in the
churchyard. The entire space dug out was now filled up with
cement concrete well rammed ; 135 tons of concrete having
been consumed in this operation. A drain was laid on the
rock bottom under the concrete to drain the springs which
continued to flow in from the north side of the building. A
finer concrete was spread upon the surface between the piers
under the tower arches, and upon this a bed of cement eighteen
inches wide was floated off to a level to take the centerings.

" The shorings to each arch are constructed in two separate
portions, the lower portion on ' tressel ' and the upper portion
on centre proper. This system has been adopted to facilitate
' wedging up ' or ' striking ' the centres when and where required.
The exact outline of each arch was obtained by ' scribing ' the
soffit of the inner member of the arch to which the centre was
to fit, on a skeleton template of f -inch boarding, sufficiently
wide to include the whole curve of the arch, which template
was securely fixed against the sides of the arch during the
scribing. This template was shaped by the line so scribed, and
the permanent framing worked to it ; thus the centres when
fixed fitted accurately ah1 the irregularities of the arches. The
timber used in the shoring generally is from 10 inches to 12

ON THE SHORING OF MEDIAEVAL BUILDINGS. 55

inches square, some having been selected 14 inches wide to
allow of getting out the curved backs without reducing the
working section of the timber below 10 inches by 10 inches.

" All the joints in the frames are tenoned (see Plate VII.), the
tenons being 2 inches thick in the centre of each piece, and from
2£ inches to 3 inches deep ; the joints are all shown on the
drawings precisely as they were executed. The framework was
fitted together on the nave floor first, and having been num-
bered at the joints was knocked to pieces to facilitate the removal
and re-erection under the tower. Each tressel was afterwards
built up in its proper place, and when the three tressels were
securely fixed in their respective archways, a temporary scaf-
folding was erected on them to make a platform for the putting
together and hoisting of the centres. The springing piece of
each arch was laid down on its side in that arch, and the centre
framed to it and secured together with f-inch wrought-iron
dogs ; a tackle was then rigged up to the bell beams with a fall
to the floor, and each centre was thus hoisted to its proper
position under the various arches, and securely wedged up to
the soffit with oak wedges.

" In ordering the first lot of timber for this framing it was
assumed that timber in the log, with one side only sawn, would
answer every purpose required as well as timber sawn all
round ; but this proved to be a mistake, as it was found to be
an impossibility to square to the tenons, mortices, shoulders,
and bearings, without having at least three sides of every piece
sawn die square. There being no sawpits near the building,
this timber was squared with adzes and planes where required,
causing some loss of time ; but the next consignment of timber
having three sides sawn square, much facilitated the work of
fitting together and makes much better work in every way.

"Three arches having been shored up with centering, as
described, the fourth arch was treated as follows : a hole about
eighteen inches square was knocked through the tower wall
over the apex of the arch, and about two feet above it, to allow
sufficient head room for the introduction of a hammered stone

56 SHORING AND UNDERPINNING.

discharging arch over the wrought stone arch. Two more
holes were knocked through the wall of the same size, about 2 feet
lower down on either side,* about halfway between the centre
of the arch and the transept flank walls. Three holes were
thus made to take needles at distances of about 4£ feet apart.

" Needles 12 inches by 12 inches were inserted through these
holes and supported by uprights inclining inwards at the top,
and stiffened at the height of every 5 feet by means of straining
pieces secured with dog irons. The walling over the needles was
pinned up, and wedged in every case with flat stones bedded in
cement ; and when the cement had set, a temporary centre was
fixed under the arch, the key removed, and all the arch stones
safely taken down one by one ; one-half the piers on either side
were also removed, and the entire space occupied by the arch and
piers cleared away to allow of the erection of the new work."t

* The arrangement in the Drawing on PI. VIII., which shows lintels
inserted over the needles, and the needles themselves all on the same level,
is considered by Mr. Seddon to be better than that actually executed and
here described. The framing also is shown as fitted to a restored arch, it
having been found impracticable to delineate the actual distortion of the
piers and arches.

f The following details of the weight thus carried, with the calculations
as to the manner in which it was distributed, and the breaking weights of
the several portions of the timber framings employed, are appended by Mr.
Seddon at the end of his paper : —

By actual experiment, ashlar in spire is found to weigh 1*527 cwt. per

cub. ft.

„ „ rubble masonry in tower weighs 1-33 cwt. per

cub. ft.

There are in spire 2534 cub. ft., weight (at 1-527 cwt. per cub. ft.) = 3869-418
cwt. = 193-47 tons.

There are in tower 9-016 cub. ft., weight (at 1-33 cwt. per cub. f t.) =
11991-28 cwt. = 599-564 tons.

There are six bells, framing and floor, weighing about 5 tons.

Total weight at arch springings = 798-034 tons.

There is no discharging arch over tower arches. Actual working sectional
area of each arch, 3-45 ft. super. Many stones fractured.

Sectional area of each pier, wrought facing, 8 '34 ft. super. ; rubble core,
9-66 ft. super. : total area, 18 ft. super.

Weight on each pier, 199-5 tons = 11-08 tons to the ft. super. On failure

ON THE SHORING OF MEDIAEVAL BUILDINGS. 57

The three remaining arches were afterwards similarly restored,
and the tower and spire now stand upon a base which will
remain immovable for all future time.

The most important lesson to be learnt from this example is
the same as that taught us by the memorable fall of the tower
and spire of Chichester in February, 1861, viz., that when a
heavy load is to be placed upon piers and arches, it is madness
to build the piers in rubble masonry with ashlar work only as
a casing. The piers which are to carry such a load should be
built in ashlar or dressed stone-work throughout their entire
thickness, as was done by Mr. Scott (afterwards Sir Gilbert) in
the rebuilding of the piers at Chichester. If such a method is
found to be impracticable on account of its cost, the core of
rubble masonry must at all events be built in cement, so that
there can be no possibility of its settling down and leaving the
weight to be carried by the casing only.

The method of restoration adopted at Grosmont may be
briefly recapitulated as follows : — All the four arches and piers
being unsafe, it was determined to restore them, i. e. replace
them in new work ; there being only funds enough to admit of
the restoration of one arch, it was decided to restore that arch
which, with its piers, was found to be in the most dangerous
condition, consequently the wall above this arch (the north
arch) was needled, and the other three arches were temporarily
centred to prevent their falling before they could be attended
to in their turn. The arch under the needles was then taken

of the rubble coring, the ashlar facing doing duty for the whole pier carried
23 P92 tons, and was crushed.

Actual total weight per ft. square on foundation, 11-71 tons.

Breaking weight of the three needles, 216 tons ; weight of one side of tower
at level of needles, 170 tons ; estimated actual weight of the needling, 70 to
75 tons. (The corbelling to octagon, with arching over, throws from 40 to 50
tops on each quoin N.W. and N. E. These quoins rest on the parts of the
piers allowed to stand.) The load on the needling being only temporary, a
co-efficient of safety of only 3 was adopted.

Actual breaking weight of each warped-up centre, 1050 tons ; weight on each
199'5 tons ; safe working permanent load, 210 tons : co-efficient of safety, 5.

58 SHORING AND UNDERPINNING.

down and half the piers on either side, to be replaced in new
work set in cement. Now it must be remembered that although
the north wall of the tower was carried on the needles and the
east and west arches had centres under them which would only
break under a load of 1,050 tons, yet the north-east and north-
west quoins of the tower (which were estimated by Mr. Seddon
to weigh 50 tons each) had nothing to carry them but the
portions of the piers which were allowed to remain standing ;
these must in this case have been strong enough to carry
this weight ; but the reader's attention is drawn to this point
because, in many instances of similar restoration, the core of
rubble work may be so decayed as to be utterly incapable of
bearing the quoins, even for so short a time as would be neces-
sary ; in such a case the quoins themselves, and all the four
walls round the tower, must be shored with needles and posts,
so as to take as much of the superincumbent weight off the
piers as possible : it is a mistake to suppose that centering
under the arches entirely relieves the piers of their load.

The reader will have noticed in Mr. Seddon's paper that the
idea suggested itself of restoring the crushed piers and arches
of the tower at Grosmont by taking out a damaged stone here
and there, and replacing the stones so removed with other
sound stones ; this method, though found to be impracticable
in this case because of the piers being so much out of the
perpendicular, has still been carried out with complete success
in many other cases. But when such a method is adopted, the
greatest care must be taken that the piers to be recased are
first almost entirely relieved of their load by shores and centres,
and that only small portions are renewed at a time.

These old Gothic buildings require the most gentle handling ;
for if they have once been damaged by fire or storm, or if at
any previous time they have sunk down or become distorted, it
takes very little to upset the state of equilibrium into which
they have settled, though they may have remained in that
state for hundreds of years. The fall of the tower at Chichester,
though hastened by the storm of wind which raged during the

ON THE SHORING OF MEDIEVAL BUILDINGS. 59

night before the catastrophe, was no doubt originally brought
about by the disturbance caused to the equilibrium of the piers
by the removal of the organ screen which spanned the nave at
their feet, and also by the manner in which it was attempted
to recase the piers after their dangerous condition had been
discovered. So, whenever it is necessary to make any alteration
or to underpin a building, every possible precaution must be
taken that the equilibrium is not upset or the building shaken.
In a speech delivered at the Institute of British Architects, and
recorded in their " Transactions " at the time of the Chichester
disaster, Sir Gilbert Scott thus describes the work that was
carried out under his supervision at the Church of St. Mary at
Stafford, which is an example of the stone-by-stone method of
restoration : —

"The first operation," he says, "was to bind the tower
round (internally) with very strong iron bars, with right and
left screws, which were screwed up as tight as possible. This
was done at two different levels. We then dug round the
base of the tower as low, at least, as the bottom of the founda-
tions, removed the remains from all surrounding graves (which
had, as is too often the case, done very great mischief, being
much deeper than the foundations), and filled up the whole space
with a solid mass of concrete. Having shored the arches and
the piers, so as to carry as much as possible of the superincum-
bent weight, we began gradually to remove the loose stone-
work and to put in new (or rather additional) foundations,
spreading out upon the new concrete. This operation requires
a system of movable shoring quite distinct from the more
permanent shoring already mentioned. This secondary shoring
is continually being moved upwards as the work proceeds, no
part of the old work being taken out at one time beyond what
is necessary to give room for the insertion of the new portions
actually in hand at the time. In each course, or at short inter-
vals in the height, we inserted chain bars (which are best of
copper) in short lengths, but so constructed as eventually to
form continuous ties all round the pier. In effecting these

60 SHORING AND UNDERPINNING.

operations, I was brought to the conclusion that it is impossible
to exaggerate the danger and the difficulty that exists in pro-
viding shoring of sufficient strength ; for in this, as in every
work of the kind in which I have been engaged, I found that
all the shoring that I could by any possibility get in was only
barely sufficient for the purpose. I have seen enormous timbers
bend under the pressure to which they have been subjected, and
I wish to offer my most decided opinion that in most cases it is
absolutely necessary, before a single stone is removed, to insert
all the shorings which can be brought to bear within the space
to be operated upon. I would also advise that in no case should
the shores be half timbers, or timbers of an oblong section, but
that they should be of square or round timbers, so as to have no
tendency to bend in one direction more than in another (in
large works, indeed, the shores must be larger than single
timbers). In one case (a minor work) which I had in hand, I had
expressly provided for the use of whole timbers in the specifica-
tions ; but the clerk of the works had permitted half-timbers to
be used, and the consequence was that the shoring gave way
very perceptibly, much to the detriment of the work. Another
precaution I would recommend is the use of the hardest stones
which can be procured, for if this be neglected, the new work
is almost sure, when the shoring is removed, and the weight
brought to bear upon it, to split ; and it is needless to say that
cracks in such supplemented masonry are far more dangerous
than in a new structure, for by throwing the weight upon the
old core (if any remains), or upon piers not yet repaired, or upon
other old work, such partial failure of the new stonework may
lead to the most serious consequences. Under no circumstances,
therefore, should anything approaching a soft stone be made use
of, whatever may be the materials of the old pier. The next
thing I would urge is the avoiding of ordinary lime mortar, and
the use of cement. Besides setting the new work and pinning
it in cement, it has been my practice to run the core behind
with liquid cement, first pouring in water and then the cement
grout, which, when thus used, I have found in some cases to

ON THE SHORING OF MEDIEVAL BUILDINGS. 61

penetrate the interstices to a depth of nine to ten feet below
the level at which it was poured in, as if it were so much quick-
silver. While engaged upon these works on one occasion a loud
report was heard by the workmen, like the report of a gun, and
it was found that one of the pillars of the chancel (quite uncon-
nected with the tower) had split almost from bottom to top,
owing to some indirect pressure brought upon it by the opera-
tions going on at the tower, which shows that the shoring
should not be limited to the tower itself, but should in some
degree be extended to adjoining parts. The shoring should
have in all cases a special foundation provided for it. The floor
of the church is wholly insufficient for the purpose, and it would
be a fatal error to trust to it."

The reparation of the tower of Hereford Cathedral by the
late Mr. Cottingham, though on a much larger scale, was in all
other respects nearly identical with Sir Gilbert Scott's work at
Stafford, which has just been described. In all these examples,
a good foundation of concrete for the shores was of paramount
importance. M. Flachat, before he erected the mammoth shoring
which held up the tower and lantern over the crossing at Bayeux
Cathedral, sank around the feet of the four piers to a hard
stratum no less than twenty wrought-iron tubes of 4 feet
diameter internally, which he filled up with concrete; and upon
these tubes, and between the foundations of the piers, he laid a
bed of concrete 9 feet thick, the top of the tubes entering 3 feet
into this concrete. The weight of the shores and the tower
which they had to carry was of course very considerable, and
fully justified the extreme caution taken with these foundations.

A brief description of this work is thus given by Mr. Burnell
in a paper published in the " Transactions of the Eoyal Institute
of British Architects " : — " Upon the concrete bed," described
above, " M. Flachat erected a double set of frames of whole
timbers on either side of the centres originally placed to support
the arch (before M. Flachat was called in), for the purpose
of forming a seating for a set of needles carried upon a series of
screw-jacks, and made to support the masonry of the square

62 SHORING AND UNDERPINNING.

part of the tower, a little above the vaulting of the nave and
transept. The tower was carefully hooped with iron bars keyed
up whilst they were still hot, so that their shrinkage actually
closed the masonry which had previously been fissured over the
openings ; and before altering the centres to the form M. Flachat
thought requisite, he also surrounded the springings of the
arches of the nave with a strong wrought-iron cradle, intended
to resist the lateral thrust. The centres were then strengthened
and modified so as to allow the easy underpinning of the piers ;
and the lateral arches of the nave, choir, and transepts, which
had participated in the movements of the piers of the tower,
were carefully shored up. Every precaution was taken to
protect the original mouldings of the vaulting, and the sculp-
ture of the capitals, columns, and bases, by enclosing them with
rubble masonry, against which the shores were made to act
directly." (It should be mentioned that the four quoins of the
tower were needled, by a system of needle shoring totally in-
dependent of that already described, on each side of the centres
of the great arches. The needles carrying the quoins were each
made of three wrought-iron girders bolted together with four
timber flitches, forming one exceedingly strong beam ; four of
these needles lying across each other, and forming a square on
plan, were inserted just under the neckings of the caps, at the
top of each of the four piers, and were carried by sixteen massive
posts, each made of nine whole timbers strongly bolted together,
in the same manner in which the masts of a ship are constructed.
These posts were each 16 metres high (52-52 feet), and con-
ducted the weight of the quoins straight down to the bed of
concrete on the ground.) " It is to be observed that the needling
was totally independent of the centres of the great arches, and
was designed solely to support the weight of the tower and
octagon above the line of the vaulting ; the arches and the
spandril fillings were all that bore directly upon the centres
themselves."

When all this shoring was erected (and it completely filled
up the crossing, being braced across and across the space), the

PLATE. K.

JAkerman.aoto-M.Londo

CMacUn Stock 3d

ON THE SHORING OF MEDLEVAL BUILDINGS. 63

piers were entirely removed from under the tower, and rebuilt
from their foundations, and the arches were restored by the
stone-by-stone method.

The cost of this work was 32,220Z. M. Viollet-le-Duc, whose
opinion was consulted before it was decided how the tower
should be treated, gave it as his opinion that the simplest and
cheapest plan was to pull it down altogether and rebuild it
from its foundation, and he estimated the cost of this at a sum
considerably smaller than 32,220Z. But even if M. Viollet-le-Duc
was right, there will always be a satisfaction, to archaeologists
at least, that the original tower was preserved intact. On the
top of the Gothic lantern there was an Italian Eenaissance
dome, which was removed before the tower was underpinned.

The success of this operation is, however, considerably marred
when it is compared with the stone-by-stone underpinning of
the tower of Hereford Cathedral by Mr. Cottingham, where,
although the weight underpinned was double that at Bayeux,
the money expended was less than one-fourth. The weight of
the spire at Chichester was also nearly double the weight of
the tower at Bayeux, and as it would have been necessary to
employ the same method to restore it satisfactorily as was used
at Bayeux, on account of the rottenness of the piers, it is
perhaps as well from an economical point of view that it fell
down, especially as not a life was lost nor a limb broken.

The cost of rebuilding it was in round numbers 50,000?., and
had it been underpinned as the Bayeux tower, the operation
would probably have cost much more than this.

We will now go on to consider the suppositional cases of
underpinning depicted on Plates IX. and X., which are copied
from M. Viollet-le-Duc's Dictionary under the word "Etai."
The following is a synopsis of the treatment of these cases
described in that work.

Taking the first case on Plate IX., the cylindrical column A,
which carries vaulting ribs in all directions, and one or two
stages of similar columns above, has become crushed under the
load, as shown in the sketch. In order to enable the damaged

64 SHORING AND UNDERPINNING.

stones to be removed we must construct a square frame of oak,
as indicated in the sketch, B in perspective, and in B' in plan,
with sides tenoned into gaping mortices, into which wedges are
driven at C, which with the bolts b insure the frames being
fitted tightly against the face of the cylinder. This frame is
fitted (as at C, in the sketch D) under the necking of the
capital, and is carried by eight stout posts G, inclined suffi-
ciently to allow the new stones which replace the old at K H to
pass in freely. Should there be any sound stones below the
necking, four wrought-iron straps must be screwed, as shown in
the sketch F, to the sides of the frame, and their feet L inserted
in the joint, to catch the under side of the last sound stone ; the
rest of the column can then be removed and replaced in new
work.

If the whole of this lower column is crushed, together with
the springing stones of the vault, the vaulting ribs must be
centred, and the column above must be treated in the way we
have just described for the lower column, the eight posts passing
through the vaulting panels to the ground below.

The second case depicted on Plate X, is a neat application of
the principle of needle shoring. A pier E, which carries two
main arches A', two diagonal arches B', and one transverse
arch C', as well as the weight of the upper vault, has become
crushed under the load. In this case, where it will be necessary
to use so many timbers in so small a space, we must take care
to arrange them so that they will not interfere with the building
of the new work. " To shore is nothing, but to shore in such a
way that one can build between the shores is often a difficult
problem to solve." The transverse and diagonal arches having
been centred, the two main arches should be supported as
shown at A in the elevation, and the springing stones of the
arches from I to K, which have shared in the ruin of the pier,
can then be taken out and notches cut to receive the needles
at L L. The needles, in order to occupy as small a space as
possible, are each made of four strong pieces of wrought iron,
bound together with hoops as shown at M ; they are made to

PL ATE, X.

C.Hcukn Stock D«l

J. AJtenom Photo .litK.Lonaon.W. C .

ON THE SHOEING OF MEDIAEVAL BUILDINGS. 65

rest upon strong pieces of oak at 0 in the elevation and 0' on
the plan, and are carried by the four stout posts N and N'. It
will also be necessary to support the wall above the needles
with raking shores at H and H'.

When the old work has been removed and the new work
finished, the posts and needles should be taken down first, then
the centres under the arches, and last of all the raking shores
at H and H'.

(66 )

CHAPTER VII.
ON THE MECHANICS OF BAKING SHOEES.*

WE will suppose C B (Plate XI.) to represent the section of a
wall that requires to be supported by the raking shore A B,
resting on the ground at A ; AC being the ground line. Let
there be a horizontal force T near the top of the wall at d,
tending to overturn it about its bottom edge C ; the moment of
this force, which measures its tendency to overturn the wall, is —

T x Cd.

This is resisted by the weight of the wall (W) acting vertically
at its centre, and having a moment about C of

W x Ce,

where C e is generally half the thickness (t) of the wall. When
these forces just balance, the wall will be about to fall over, and
the two moments will be equal ; therefore —

T x Cd= W x Ge.

Now, in order to restore the wall to its original condition
before the force T acted upon it, we must find some means of
completely balancing this force, and this can be done by placing
the shore A B against the wall at B, where it is firmly fixed
against a plank or walling piece, by means of a needle driven
through both the plarik and the wall ; then by wedging up the
base A, a horizontal pressure (Q) is produced against the wall,
such that the moment of Q about C balances that of T, or

Q x BC = T x Gd
= W x Ge

- n W x * rn

••Q== 2B-C"

* Copied from an article in the Building Xews.

ON THE MECHANICS OF RAKING SHORES. 67

In this formula, B C and t should be expressed in feet, W and
Q in cwts. If the shore presses against B with a horizontal
force Q, there must also be a reaction of the wall against the
shore equal and opposite to Q, so that Q represents a horizontal
pressure against the head of the shore.

In order that the raking shore may have its full effect in
counteracting the outward thrust or reaction Q, it is essential
that it should be prevented from sliding upwards by having a
sufficient weight of wall above B, so that when the pressure Q
comes upon it, the head of the needle may be kept immovable
by means of the superincumbent load. If, therefore, the top
of the shore is put very high up against the wall, it will be of
little service in preventing it from being overturned. Let P be
the vertical pressure necessary to resist a horizontal thrust out-
wards, equal to Q at B, and w the weight of the shore itself
acting at its centre g. Then the sum of the moments of P and
w, about A, the base of the shore, must balance the moment of
Q about that point ; therefore, we have —

QxA3 = (PxAC)+ v££>

B q being a horizontal line meeting a vertical from A at q. This
equation may be put into the form —

Qsin. 0 = (P+*f)cos.0;
\ 2/

0 being the angle BAG which the shore makes with the
horizontal ; and from this we obtain —

P = Q tan. 0 - ~ . . . . (II.)
2

So that when Q and w are known, and also the angle of inclina-
tion of the shore, we can find from this equation what vertical
pressure (P) must be brought to bear on the head of the shore,
in order to keep it in its place when the force Q tends to thrust
it out. If the value of P is known beforehand, we can also find

08 SHORING AND UNDERPINNING.

what amount of horizontal force (Q) it will be able to counter-
act; for

The horizontal and vertical forces at B being thus determined,
we can find the compression (F) down the shore by resolving
P and Q, in the direction of A B, and adding their resolved parts
together; therefore, we have —

F = P sin. 6 + Q cos. 0 . . . . (IV.)

In order to find whether the shore is strong enough to resist
this compression, we must use the formula for a long pillar,
namely

L = a x

Where a = 15*5 for fir, d is the diameter or width in inches
and I the length in feet ; L being the safe load in cwt. that
may be put on the pillar. As, however, the depth of a shore is
usually double its width, we shall get twice the resistance, as
obtained by the above formula, or F should not exceed —

Safe load = 31 x j • • . . (V.)

There will also be produced a cross-strain, S, acting at right
angles to the shore, and tending to bend it inwards, which is
equal to the resolved parts of P, Q, and w — namely,
P cos. 0, Q sin. 0, w cos. 6.

And since these strains are uniformly distributed over the entire
length of A B, the total amount of cross-strain at the centre is
equal to half their sum, or

S = i {Q sin. 6 + (P + w) cos. 0}.
If wre substitute for P its value as found from (II.), we have

S = Qsin. 0 + -cos.0. . . .(VI.)

ON THE MECHANICS OF RAKING SHORES. 69

To find the deflection (D) in the middle which the cross-strain
S will produce on a heam of fir, we use the formula —

The dimensions, D, b, and d, being in inches, and Z in feet ; S is
to be expressed in cwfc. If the value of D thus obtained is an
appreciable quantity, it will be advisable to counteract the cross -
strain by a strut g h, so as to prevent the resisting power of the
shore from being impaired ; and the force S will represent the
compression down this strut. If we wish to find what ratio S
bears to the breaking-weight of the shore we can use the
formula —

Breaking-weight = 3-2 x ^-j^ .... (VIII.)

b and d being in inches, and I in feet ; the breaking-weight is
found in cwt. The strain S must not exceed one-sixth of the
breaking-weight thus obtained.

We can now determine the magnitude and direction of the
resultant (E) of all the forces, its point of action being at the
base A of the shore. Suppose this resultant to make the angle <£
with the horizontal A C, then by the rules of mechanics we have

E . cos. <f> = Q

E . sin. <£ = P + w

from (II.) = Q . tan. 0 + ~"

But, E = E ysin. *<£ + cos. 2<£

.-. E = v/Q2 + (P + w? . . . . (IX.)

from which we obtain the magnitude of the resultant E. To find
the direction of E or the value of the angle <£, we have

E . sin. <£ P + w
tan.4> = -E>cog ^ = — Q—

= tan. * + . . . . (X.)

70 SHORING AND UNDERPINNING!.

This last formula shows us that the greater we make the
horizontal force Q, the more nearly will the angles <£ and 6
approach to equality, or the direction of E get nearer and

nearer to that of the shore, for the quantity -^~ diminishes with

2 y

the increase of Q. The minimum value that Q can have is
when P is nothing, or the head of the shore merely rests against
the wall, and is not pressed upon by any vertical force, in which
case we find from (III.) that the value of Q is

and substituting this value for Q in (X.) we obtain
tan. <£ = 2 tan. 0.

In this case, therefore, the direction of the resultant becomes
that of the line A E. We see, then, that the resultant force E
may have any direction between A E and A B, according to the
amount of the pressure Q ; but it will generally lie nearer to
A B than to A E, and consequently it is advisable to have the
abutment at A very nearly at right angles to the shore A B, in
order that any horizontal thrust at A may be counteracted by
the resistance of the earth.

Example. — We will now show the practical application of
these ten formulae, by taking the case of a brick-and-a-half wall,
40 feet high and 10 feet frontage, supported by a raking shore
of fir 12 inches by 6 inches, the top of which is 30 feet above
the base of the wall, and its spread at the foot 6 feet. The
angle 6, or B A C, will be 78° 41', tan. 0 = 5, cos. 0 = -19623,
sin. & = -98056, and the weight w of the shore is 4'5 cwt.
Taking the wall at 1 cwt. per cubic foot, its W will be 467 cwt.,
its thickness t being £ of a foot.

We first find the maximum horizontal thrust Q from (I.)

W . t 467 x x

60 = 9 cwt'

ON THE MECHANICS OF RAKING SHORES. 71

The vertical pressure P, which Q produces, is obtained from
(II.),

P = Qtan. 0 - ~

= (9 x 5) - 21 = 43 cwt., nearly.

This is the least value of the pressure upon the top of the shore
that will counteract the outward thrust Q ; but, as in this case,
the actual weight of wall above B is 117 cwt., or nearly three
times as much as the above value of P, we see that there is but
little danger of the shore being pushed out by Q, provided it is
tightly wedged up at A and B, as the shore cannot be turned
about the base A without its head being lifted up, which would
cause the needle to rise, and also the wall above it. For, if we
put P' = 117, we find from (III.) the value of Q' necessary to
make the shore lift this load,

which is more than 2£ times the maximum value of Q as given
above.

The horizontal and vertical forces (P, Q) being known, we
can find the compression F which they produce on the shore in
the direction of its length from (IV.),

F = P . sin. 6 + Q . cos. 0

= (43 x -98056) + (9 x -19623)
= 44 cwt.

From (V.) we can ascertain what is the safe load that such a
pillar will sustain, the length being 3O6 feet, and the diameter
6 inches ;

Safe load = 31 x ^

which agrees very nearly with the value of F obtained above.

72 SHORING AND UNDERPINNING.

The cross-strain S produced at the middle of the shore, and
acting at right angles to its depth, is found from (VI.)

S = Q.sin. 0 + ^cos. 6

= (9 x -98056) + (1 • 12 x -19623)

= 9 cwt.

From (VII.) we can find the deflection which this strain of
9 cwt. will cause at the middle of the beam,

9

In this case, as there is a deflection of nearly 1 inch at the
middle of the shore, it will be advisable to introduce a strut g h
otherwise its resisting power as a pillar will be impaired. The
compression down the strut will be the above value of S, or
9 cwt.

The breaking- weight at the middle of the shore may be found
from (VIII.)—

Breaking-weight = 3-2 x ^f

which is ten times the strain S, which we have just obtained.

The pressure which the resultant force K exerts on the base
A can be calculated from (IX.) —

K '

= V92 + (45 + 2-25)2
= 48 cwt,

The direction in which this force E acts at A, or the angle </>,
which it makes with AC, is found from (X) —

w
tan. </» = tan. 6 + ^Q

= 5 + t5 = 5-25,

PLATE XI.

ON THE MECHANICS OF RAKING SHORES. 73

By referring to a table of natural tangents, we find that 5'25
is the tangent of 79° 13', so that the direction of E makes
an angle of only half a degree with the shore itself, when Q
presses with its maximum force against the head of the shore.

When P is nothing, the direction of the resultant is AE, and
tan. <£ = 2 tan. 0 = 10, in which case the angle E A G = 84° 18' ;
the value of the angle </>, therefore, will in any case lie between
79° and 84°, according to the amount of the reaction (Q) at B.

The above example, it should be borne in mind, is taken for
the case of one shore only in a system of raking shores ; but
when two or more shores are erected against a wall in the same
perpendicular plane, each shore must be considered as resisting
the outward thrust of its own portion of the wall only, and a
separate value of Q must be found for each of them.

It is perhaps needless to say that in practice it would not be
necessary to make use of all the formulae which have been
proved and demonstrated in this chapter, but for the sake of
those who are fond of mathematical investigation the whole
science has been laid down in its completeness. An example
which shows the application of the more useful of these formulae
has already been quoted at the end of the chapter on raking
shores, and it will be found that the rules there given will be
all that are really necessary in actual practice.

In conclusion, I must apologise to my readers for the some-
what condensed form in which the proofs of the several formulae
are worked out ; but as I had previously stated that this
chapter is only intended for those who are well acquainted
with the science of Trigonometry and Statics, I concluded that
any more elaborate explanation of the way of arriving at the
different steps in the proofs would be unnecessary.

INDEX.

ANGLES of raking shores, 11

BAYEUX CATHEDRAL, shoring at, 61
Best wood for shores, 17
Braces, 7

CHICHESTER tower and spire, failure of, 57, 58
Cross strain on raking shores, 8

DANGEROUS structures, 24

,, ,, clauses of London Building Act relating to, 37

Distance apart of systems of raking shores, 10
Dogs, iron, 10

EXPANSION of cement, 27

FAILURE of foundations, 25, 31

Flying shores, 19

Forces acting on shores, 5, 66

Formula for pressure on flying shores, 20

Formulae for needle shoring, 36

,, pressure on raking shores, 12, 66

GROSMONT CHURCH, shoring of, 45

HEAD of shore, position of, 6
Horizontal shores, 19

JOGGLE, 4

LEVERING foot of shores, 6

Lime concrete in underpinning, 28

MECHANICS of raking shores, 66
Mediaeval buildings, shoring of, 44, 63

7G INDEX.

NEEDLE SHORING, 23, 24

Needles, 4, 10, 32

Number of shores in systems, 9

OBJECTIONS by "neighbours to flying shores, 21

POSITION of head of shore, 6
,, needles, 32

BAKING shore, simplest form of, 4

,, shores, systems of, 7
Reinstatement of leaning walls, 16, 17
Responsibility of architects, 33
Eider shores, 8

SCANTLINGS of flying shores, 21
,, raking shores, 9

Scarfs in raking shores, 9
Shores, numbers in systems, 9
Shoring of mediaeval buildings, 44, G3
Simplest form of raking shore, 4
Sole piece, 4, 11

Stafford, shoring at St. Mary's Church, 59
Systems of raking shores, 7

TOSSLE, 4

Trussing of raking shores, 9
UNDERPINNING, 26

VARIETIES of raking shores, 15
Variety of flying shore, 22

WEDGES, uses and limitations of, 9
Wood, best for shores, 17

THE END.

BRADBURY, ACNF.W, &, CO. I.D. , PRINTERS, LONDON AND TONBR1DCE.

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